The club I'm a resident singer at on a Sunday have a policy on guest nights where only one of the four residents (ranging from me as a solo singer, two duos - one male/female and the other male/male, and a four pice harmony quartet) do the support. Four songs in the first half and either three or four in the second half depending on time left. The guest then gets at least 40 minutes per half. The resident for each night is chosen to contrast the guest where possible so I wouldn't do the support for Harvey Andrews say. It doesn't matter who comes through the door asking for a spot they won't get one. On the other three/four nights of the month which are run as a singaround anybody can have their turn if they want to do something. It's worked for as long as the club's been running and only once have I seen someone come in expecting to sing and left because it was a guest. Maybe he didn't have enough cash to get in but I wasn't running after him to find out.

Cheers
Mark