Character-Magnetic Team

Some groups or teams act as a character magnet. The group keeps on attracting new members and/or close allies. In Shonen anime, especially, a former villain of the Worthy Opponent class tends to become a new teammate.

This is often used near the beginning of a series to build the cast. When done well, this makes a character's entrance more interesting. When done poorly, it is an Anvilicious way of adding a new character. If it's done often, it will ensure that your series will have Loads and Loads of Characters.

Sometimes, this happens to replace other characters in series with high turnover. Usually there's a Magnetic Hero at the center of the team, though other characters around him will also exert "pull" to attract newcomers.

This is the logical extension of the Debut Queue. Compare Hitchhiker Heroes, where the team is attracted to the new members. See also You ALL Share My Story for a similar phenomenon.

Anime and Manga

 * Mahou Sensei Negima is perhaps the crowned king of this trope. At the beginning of the story, it's only Negi. Asuna and Konoka are added to his group fairly quickly. The Kyoto arc adds Setsuna, Nodoka, and Asakura to the main group, while also introducing Yue, Paru, Ku Fei and Kaede. In the festival arc, Yue, Paru, Ku Fei and Kaede, along with Chisame and Kotaro. The Magic World arc adds Chachamaru as well as five ostensibly "normal" girls - Natsumi, Akira, Ako, Makie, and Yuuna. Am I forgetting anyone?
 * Sayo, Anya, Chamo.
 * Also Evangeline, after the Kyoto arc. Kind of.
 * Most recently, Jack Rakan has been tagging along with them.
 * During the attack on Gravekeeper's Palace, Mana, Takane, Mei, Collet, Beatrix and Shiori join the fray.
 * It's funny that, but still has a growing group.
 * It's even more ironic given how
 * It also seems his father's group the Ala Rubra was something of this: Rakan ended up joining after being defeated by Nagi.
 * Yu Yu Hakusho used the first arc of its plot solely to do this.
 * Bleach: Ichigo's group acts as this right the way through the manga. If characters aren't drawn fully into the group, they at least become recurring friends and allies of the group.
 * Dragon Ball is a frequent flyer of this trope, bringing in characters in this manner all throughout the first series and through Dragonball Z.
 * Pokémon, by its nature, is supposed to attract more and more "characters" over time. They do, however, get Put on a Bus or eliminated some way more frequently than not. This due to only six being allowed on a trainer's team at any one time.
 * It can get annoying when everyone except Pikachu gets sent back to Professor Oak because of Ash's selfishness. Three times and counting.
 * In GetBackers, the protagonists' villains also often return as allies. As a result, the one arc to feature a large cast is an almost all-villain team.
 * In a tolerable way, as most of the "villains" are jobbers trying to get paid, just like the heroes. In that environment, the guy you were fighting last week might be a coworker this week.
 * This occurs gradually over the course of the first half of Cowboy Bebop, much to the main character's chagrin.
 * Rurouni Kenshin does this profusely (even if the characters aren't immediately allies afterwards).
 * This happens in Mariasama ga Miteru, although it's not as bad as it could have been, since the basic team is rather picky about the persons they associate with.
 * Half of the plot of One Piece is Luffy's search for crew members for his ship.
 * The majority of the cast of Ranma 1/2 came on like this. Especially the shonen trope of being enemies first.
 * Although most of them were also enemies after, at least to someone. Mostly Ranma's and Akane's suitors are allies to their respective loves, and bitter enemies to any of their rivals.
 * Ditto for Lyrical Nanoha. This was more or less the only way they added cast members up until StrikerS, and they were still doing it then, also. They don't call it "befriending" for nothing.
 * Bobobo-bo Bo-bobo is inexplicably able to call upon former foes (the crazier ones, at least) to aid him in battle, or at least act as cannon fodder.
 * Rave Master starts adding new members in the second volume, and finishes up it accumilation of team mates in volume 28. Characters range anywhere from having been met on the street to being parts of a Quirky Miniboss Squad.
 * A Certain Magical Index: Touma and Index. Justified in that one has an unusual power, and the other is sought after for her information (and thus protected) by a large part of the world.
 * Code Breaker: Similar to the To Aru example, Sakura and Ogami are unique, as one is a "Rare-Kind", while the other has the most powerful Code ability.

Comic Books

 * The X-Men and their affiliates. The sole criteria for being affiliated with the team is that you have to either be a Mutant, or involved in mutant politics in some way or other (a sympathetic human, an ex-mutant, a genetic experiment from another dimension who happened to get saved by mutants, etc). Since about 10% of the Marvel Universe's population used to be mutants at one point, this meant a great deal of snowballing, and even after the "Decimation" event had over 90% of that population Brought Down to Normal, the X-Books' cast is still larger than almost all of Marvel's other properties, put together.
 * The band of survivors in The Walking Dead runs into new people regularly, most of whom end up sticking around. This is necessary, of course, because established characters die horribly at about the same rate as new ones join.
 * One of the common criticisms of Brian Michael Bendis's Avengers run is the frequency with which characters join the team only to proceed to stand around in the background with nothing to do except interject an occassional one-liner or get punched in one panel of a team fight while the lead characters do all the heavy lifting - if they even appear in the book at all (Daredevil at one point joined the team then didn't appear again for several issues).

Literature

 * The rabbits in Richard Adams' Watership Down start out as a small group and add several other characters to their number over the course of the book.
 * In JRR Tolkien's The Fellowship of the Ring Frodo and Sam set out from the Shire alone, but collect Merry, Pippin, Aragorn, Gandalf, Gimli, Boromir and Legolas in the formation of the titular fellowship before they part ways at the end of the book.
 * And then, when they are split up, accrue everything from a Token Evil Teammate to walking trees to the Affirmative Action Girl.
 * Robert Asprin's Myth Adventures series frequently features a villain from the preceding book as an ally, resulting in the cast growing in almost all of its earlier books. Most of Skeeve's allies have wanted him dead at some point...
 * Which is subverted in , when Skeeve refuses to let the Big Bad of the book join the group because.
 * The Tanith First-And-Only Regiment of Warhammer 40,000: Gaunt's Ghosts has twice taken on a great deal of new blood to replace losses they've suffered.
 * Rogue and Wraith squadrons in the Star Wars: X-Wing novels both function this way, with pilots from lesser squadrons jockeying for positions in the group. Rogue is officially the "top gun" fighter squadron of the entire New Republic fleet, so this makes plenty of sense. Wraith... Not so much, but they do at least have a famous commander and a rep for getting stuff done (and blowing stuff up). Plenty of turnover in both groups, too.
 * In The Belgariad, this happens twice. The first time, it was done intentionally by Belgarath to "fulfill the prophecy". In The Mallorean the trope if followed correctly, even if Prophecy itself has to intervene at times.

Live-Action TV

 * Every single series of Power Rangers has the Five-Man Band joined by at least one Sixth Ranger. Some recent series start with only a Power Trio, and occasionally don't stop at six, allowing two to sometimes four Sixth Rangers. Of course, the whole reason to do this is to have more people to make action figures of.
 * Power Rangers Jungle Fury began looking like it would be a particularly bad example, as the toy line added three Rangers (based on secondary mecha from Gekiranger) to the existing five, and it was confirmed they'd appear on the show. It ultimately turned out that these three weren't new characters, but rather spiritual manifestations that the Power Trio or their respective mentors could summon into battle.
 * Power Rangers RPM narrowly avoided being an even worse example; the original Go-Onger expanded from three to five to seven Rangers, and the toy line added three more on top of that for a total of ten. Thankfully, these three stayed as toys and never appeared on the show.
 * Doctor Who gains new companions and loses old ones constantly, more so in the original series than the new one.
 * Can't forget Buffy the Vampire Slayer, especially season seven, where new potential slayers are collected every episode, along with characters like Andrew and Wood, and old recurring characters like Faith coming back.
 * Angel does that, too, to a smaller extent. At the beginning of the first season, there were three main characters with no real minor or reoccurring characters yet. By the end of the fifth, there were eight (if you count Fred and Illyria separately), as well as two dead main characters, two or three non-main characters working with the team (depending on when you count it), Conner, and more minor reoccurring characters like Knox, Nina, Anne, Conner's "parents", and a few other random Wolfram and Hart employees. The cast never got as big as Buffy's, though.
 * Both shows are notable in that they need to keep picking up new team members because old ones have a nasty habit of dying, being put out of commission or running away to the spinoff.
 * ER it is always gaining new members, if only to replace old ones. It had 100% turnover over 10 seasons—done gradually.
 * And if you've worked in a hospital, you know that's Truth in Television.
 * The OC has this thing going on where nearly every new character, even when they start out as antagonists, gets assimilated into the Cohen clan sooner or later.
 * The duo of Merlin and Arthur from Merlin have steadily accumulated a team of knights, most of which turn up in the two-part finale of the third season to help Arthur win back Camelot.

Tabletop Games

 * Common in Tabletop Games. When a player's character dies but he/she does not wish to leave the game, the group will traditionally encounter another adventurously-minded type in short order by complete coincidence.

Video Games

 * A common feature in RPGs, where the player starts out in control of one character but has a veritable army by the time they face the Big Bad.
 * Probably two of the most striking examples of this in RPGs are Chrono Cross, which has 45 characters total, and the Suikoden series, which has 108 (although, to be fair, the vast majority of the 108 are minor characters). Also notable are the Shin Megami Tensei series, while having a reasonably small amount of characters, have dozens upon dozens of Mons that may be convinced to ally with the player.
 * Heroes in SaGa Frontier can just about always run into somebody willing to join them for little reason than their own boredom. But really, being that the multiverse has a population of about 100, it's a nice thought that an actual statistically significant number of people are willing to get off their duffs and help save the world. Not quite the usual "5 plucky youths vs. the world while everyone else is busy Dying Like Animals".
 * In Fire Emblem, if there's a named character on the field, good odds point to that character either joining you or being the boss of the level. In Path of Radiance, many characters decide to join your army for the flimsiest of reasons regardless of any protestations from your characters.
 * Lampshaded in Radiant Dawn when  If you initiate a talk between him and Ike, Ike will ask him to please rejoin the enemy.
 * Sacred Stones features two paths, during which you recruit every possible character, despite said paths taking place simultaneously on opposite sides of the continent, with the path you don't choose having a Hand Wave explantion as having had a small escort, which you never see when the paths intersect later, leaving you to presume they all died.
 * The player character in Mass Effect could be considered an almost literal interpretation of this trope, seeing as, in the space between  and subsequent return, the group you had collected in your first adventure are scattered to the four winds, no longer held together by their magnet.
 * When Shepard asks Joker about the old crew, he tells him/her just as much.
 * The sequel pretty much plays it straight. Two of the characters join without hesitation once cleared of other duties, and everyone else runs that gamut from shady figure to wanted fugitives that actively despise the organisation you're working for and they'll still sign up with no hesitation.
 * Dwarf Fortress has both the titular mode, which will constantly attract new migrants as it grows, and the Adventure mode, where you can recruit more members in any town to replace those that fell to giant spiders.
 * And trolls. And troglodytes.
 * Lampshaded in Knights of the Old Republic II, where Kreia points out that your large group of followers is evidence of the power your character exerts through the Force.
 * A literal example in Planescape: Torment:

Web Comics

 * Sluggy Freelance sorta does this, as the strip started with just two characters but has added dozens to the main cast as time went on. However, since characters tend to leave just as often as they come in, the cast size at any one time is usually no more than six people.

Web Original

 * Team Kimba is still doing this in the Whateley Universe. First it was Chaka, Fey, Tennyo, Generator and Shroud, Phase, and Lancer. Then they pulled in Carmilla, then Bladedancer. Then Carmilla and Bladedancer pulled in more, to the point that both have been split off with their own teams. Now there's Vamp and the Crimson Comet.

Western Animation

 * Avatar: The Last Airbender has Aang, Katara, and Sokka first season. By the end of the series they have acquired Toph,, Suki, and several other characters they had previously encountered.