I've implemented this elsewhere as well. Charging a measly $5 for an event (and not being chuffed if you don't collect it from one or two people) is a great way to filter out the wishy washy, the stingy, and most of the people who will waste your time without providing any value.
There is a stark difference between someone willing to put >$0 (even $.25) into a group project vs not. And I don't really do business with the second anymore.