Do you use opt-in or opt-out? That is, do you agree to meet unless one or more of the group calls and says they can’t make it, or do you agree only if enough of you call and say they can make it?

With opt-out each person has insufficient incentive to make the call. If she has already decided not to go, courtesy is the only motive for informing the others. Moreover even if she is courteous, since the call could kill the meeting, if she is not 100% sure she can’t make it, she has a private incentive to wait until the last minute to make the call, just in case.

With opt-in each person has stronger incentives to try to coordinate. Because if I want to go to the meeting and I don’t make the call it might not happen.

So, returning to the question in the title, it all depends on what you want. Opt-out minimizes the chance that the meeting will be cancelled, but probably also at the expense of minimizing attendance.