In a simmilar way I was thinking about spontaneous large family (mom, dad, aunts, uncles, kids etc) Thanksgiving dinners, where the rules were come anytime the mood strikes you and eat what you want and the amount you want. Not my picture of a good time for all.
My idea is everybody bring something and agree on a list of of activities ( arrival time, greetings, snacks, dinner, deserts/drinks, visiting time, games/movies/photo albums/walks, and departure times.
This model works for family dinners why can't something like it work for something simpler but maybe more important and more intimate.
