I run a monthly singaround in my local village pub - started over four years ago and still going strong. My "rules", for what it's worth, are:

1. Everybody, from beginners onwards, is welcome.
2. You can sing or play whatever you want.
3. No amplification.
4. When it's your turn to kick something off, we'll all join in unless you'd prefer us not to, and say so.
5. Everybody gets a crack at kicking something off.
6. If you want to use a crib sheet, use one.
7. If you pass your turn, you have to buy a round (I made that one up).

My only duties as organiser are: to be there every session, to make sure everybody gets a fair share at playing, to stop people hogging the evening, to make sure the inevitable inter-tune chit-chat doesn't become interminable, and to welcome everyone and make them feel welcome.

You can see that, to a musical folky "purist", this sort of evening would not be to everyone's taste - and indeed I would admit that there are occasional longeurs - but there's still the sheer of people joining together in friendship to make music together. And you can't beat that.