It probably needs to be made clear in the way in which you advertise the club that some restrictions may apply. quality is always going to be a subjective issue. I like to think my singing is OK but self criticism is often quite difficult. Here is a link to me singing with a couple of friends who I play with on a regular basis at local clubs.
http://www.youtube.com/watch?v=FysVZlW1iMc&feature=related
Would this be above or below par for your club. How would it rate in other clubs? In my experience clubs vary widely in performance standards and ultimately I think some form of self selection is probably in operation.
john
p.s. not got the hang of blue clicky things