Optional Party Member/Playing With

Basic Trope: The player can choose if he wants certain characters to join the party.

Played Straight: After the Five-Man Band of Bob, Alice, Emily, Dave and Charlie join the party, you can pick if you want Doom Komodo, Doctor Von Teslastein, Violet, Lamia and Jason Bloodspiller to join, giving you the ability to rotate the party.
Exaggerated: You can pick who you want to be in the Five-Man Band, with the rest doing whatever they do if you don't pick them.
Justified: They offer their services, but don't have to join like the other five, and they aren't The Chosen Ones with the Famous Ancestors.
Inverted: You can choose if you want characters to leave.
Subverted: They only join temporarily.
Double Subverted: You can choose if you want them to rejoin.
Deconstructed: If you don't choose them, they die.
Reconstructed: You can get them later if you are able to bring them Back From the Dead.
Parodied: They convince you to take them along, and if you don't, they storm off in anger.
Lampshaded: "Perhaps we should take some along..."
Averted: You can't choose the party.
Enforced: The creators feel that the player should be able to choose the heroes.