emily - open, but word of mouth advertizing is a good way to keep some level of control over how a session grows. Another idea I like, is the round robin approach ... that is each particpant plays one song, and the next persos plays, and so on ... encourage all to play, but some will refrain, only wishing to play/sing along. If you make them feel OK with this, you wil begin to create an inclusive, and welcoming atmosphere ... good luck