Someone above suggested that the "better" singers will go off and form their own group. That has already happened here in Portland. The "Pub Singers" have been meeting twice a month and do not allow any books at all. That's a great idea.   The unfortunate thing is that they are exclusive: participation is by invitation only.   (After waiting several years for an invitation, I found they have some exciting, memorable sessions--but I still feel uncomfortable about their exclusivity.)                  
   Of course, they have every right to determine their own membership. In effect, every group determines their own membership--some are more open than others, some are more explicit about their content and "rules" than others.
    At this point, I think that one of the answers to this "problem" is to make the culture of each group explicit and clear to all. For instance, the second Wednesday group is a teaching-learning group: they share songs and sometimes work on songs together. The second Saturday group is open to all, but if you want a song from RUS, then someone has to know it well enough to lead it all the way through.
    Just make the content and format clear. After all, you don't want someone singing Broadway songs at a Sacred harp session. But that wouldn't happen because the name of the group clarifies what happens there.
    People just need to clarify what is acceptable or expected at each session.

David

PS    It just struck me that maybe there is a fundamental difference between people who just like to sing and those who treat singing as an art. I'm not intending to be judgmental here, just trying to draw some distinctions. Does that distinction make sense to any of you?