We always tell people who want gigs that...
1. We need to like what they do
2. We need to think that it fits in with what we do
3. We need to be confident that we can draw a crowd to make it worth their while AND our while.

The third one is the most crucial. If they agree to play on the basis of a door split and only 10 people turn up it doesn't do anyone's reputation any favours. If an unknown act promises to bring thirty friends along we simply say "sorry, we don't arrange private parties". It's tough....we reckon a potential guest needs to be able to attract people they don't already know.