Pip, in my experience "beginners" sessions become a movable feast as people pass through depending on their level of competence and interest. The group, therefore, will not be static. I think many reasonable players worth their salt would be happy to stay with a group and help beginners along. I also believe that the way to improve is by listening to, and playing with, more experienced players. Group makeup, I admit, can be difficult to determine but, if you have a list of possible tunes (such as the Lewes Tunebook, for example), and a plan for teaching/playing them each session, then it should work.

I think I would say: If you're as good as this, then come in and give us a hand!