Carry the whole group project once and you’ve volunteered for every project after it.

Capacity you demonstrate becomes capacity you get assigned. Do the whole thing once, at midnight, because the alternative was failing, and you have told the group something that cannot be untold: the deadline is covered regardless of what anybody else does. People are not cynical about this so much as responsive. They stop stepping in where somebody already has.

Decide early rather than at the end. Split the work in writing on day one, say what you will do and what you will not, and let a gap stay visible while there is still time to fill it. Visible gaps tend to get filled.