What about drawing names, a sort of lottery. Everyone who wants to go signs up. If more people sign up than spots available, names are drawn. Yes, someone may not be able to go with their friend or "clique", but maybe that's a good thing. They'd be forced to get to know their fellow members. Or, they would possibly drop out because they couldn't go with their tribe and more spots would open.
Another suggestion, if you sign up for a retreat, you also have to sign up to volunteer for some type of guild activity or job for the following year. If you don't come through on said signup you can't do the retreat the following year.