Will Fly's "rules" are short, sweet and sufficient for any event I've been to:
I started a session in my village local over 10 years ago. It's still going, now under someone else's guidance, though the "rules" I set up then apply now. They're very simple:
1. Everyone is asked to lead off on a tune or a song, in order of seat. Sometimes we go clockwise - sometimes, for fun, widdershins. No-one who wants a go gets missed out, ever.
2. Duos get two goes.
3. Unless the person whose turn it is specifically wants to play/sing a solo, and says so, anyone is free to join in. I personally encourage anyone to join in with me at all times.
The whole point of the evening is to have fun, to make music communally, to listen, entertain and learn.
No big deal.