We all our predudices and if we recognise them as such, we can laugh at our own and at others. I do think there is a danger though, when you read comments and see notices like the one that Martin mentioned above. The strength of sessions/jams/singarounds etc is that you cannot predict their outcome and that is also also their weakness. The only course of action open to you if you do not like: the amount of instruments, the type, the volume, the standard of play, the type of music, the way it is organised, or any of a number of other complaints, is to leave.

Any attempt to 'lay down the law' before the event will inevitably lessen the spontaneous approach that the best sessions would seem to to require.

All that is required I would suggest are the following-

Basic good manners.

Recognition that the music is more important than musicians.

Play only when you feel you are adding to the music, no matter what your standard of play is.

The more people you can actively involve the better, no matter what their stardard of play is.

That there is a place for perfectly constructed and rehearsed pieces of music, but that sessions are for capturing something else.