When I was in Syracuse there was a folk related concert and someone got up and said he wanted to start a group to get together and sing and to come sign a list if you were interested. If there is any kind of folk event in you r area, this would be a good place to ask for interest. An add in the paper sounds risky. You could try to get interviewed in the paper instead. Or post a sign at the college campus, or food coop, or wherever you think folkies might be. The format can be made to suit the group, but try to exert some kind of control to prevent one person from monopolizing it. Good luck!