Ebbie, it sounds like the primary purpose of your group is to facilitate music in a small community. It that's the case, then perhaps you have to forget about 'quality control' in the interests of encouraging as many as possible to play.

However as well as satisfying yourself and the performers you also have to satisfy the audience. You have to find a way to explain that philosphy to them. If they buy into it, then you're on a winner. If not, then you may have to be selective and only book performers by invitation after having heard them play.   There's no reason you shouldn't give improvers a chance to do a couple of songs, perhaps even have part of the evening as an open mic. However allowing all-comers a 20 minute slot regardless of their ability risks losing your audience.