1. Take your turn.2. Be in tune.
3. Do NOT play along unless invited to. (different situation than a jam session)
4. If folks Do play along and you don't know the chords to the song just listen. (you'll win a HUGE amount of respect from the folks who respect the music)
5. Try to sing songs with easily sung choruses (especially if you're new)
6. Understand that there are often two schools of thought at song circles. People with good social skills make a song circle fun. Those without, are aggressive, play loudly (and often badly) or seem to never sing without "THE BOOK", and as surely as night becomes day, will eventually drive the subtler ones away. I haven't seen this happen "many" times, I've seen it happen "every" time.
7. Take the time to memorize a song so you can "live it". Some folks have a devil of a time memorizing, but others just don't appear to have tried hard enough.
8. Beware of getting SERIOUS FLACK for advocating #s 1 through 8.
Shrug it off however. Music, when sung and played with "commitment and investment" (not virtuosity) is wonderful, truly a tonic for the soul....you'll see.
Rick