We host a monthly open mic/jam session on the second Sunday in the month here in Fond du Lac, Wisconsin. We have tried various formats including large groups which are really sing-a-longs. The smaller groups are more fun for the extremes: the serious musician because more soloing and harmonizing goes on and for the novice because of the lack of intimidation that can accompany a large group.Our House Concert series is just getting off the ground and so is too new to say much about except that it's a really fine way to enjoy a night of music without spending megabucks.
I also have attended a bluegrass jam in a teeny little town north of Milwaukee where they start the jam by lighting a candle beneath a picture of Bill Monroe. :-) They are serious bluegrass pickers and be prepared to solo and lead a tune!
Cheers,
Steve