It sounds like the Club members think they're on a Stay Over. sisterofmercy02 Do you own Growing Together? Is their behavior consistent with house guests on a Stay Over? If you take your sim to another lot without them and come back, are they still there?
I don't own Growing Together. When my vampire sim invites his offspring over for a club meeting, this is how the end of gathering usually plays out: 1-2 of them leave immediately, 2-3 are in the middle of some sort of task (a club activity or attending to their needs) and will only leave after they finish that task, maybe one of them will continue to hang out for a while as though they were asked to hang out, but not for an entire day.