There are damn few places where people can learn to play music in an ensemble. Rule of thumb, if you are going to accompany someone, make that person look good. If you can help improve their performance, you might ask permission to accompany them. Unaccompanied ballads can be accompanied by being aware of the rhythm or tempo or rubato supplied by the singer. If there is not a tempo or rhythm in the vocal, rubato chords can sometimes enhance a song if you know that song in advance.

Musical sensitivity is an acquired characteristic. It takes training and discipline. A good accompanist is a good musical editor. (Knows the notes to leave out.)

There is something to be said for the group dynamic. I find that if I'm in a group that is not sensitive to the dynamics or the vibes in the room, I won't participate. I'll listen. I tend not to like situations that are highly structured by a group leader. I like the kind of environment whereby someone can say "that song reminds me of another song". Bottom line. No ettiquette, sensitivity to the room "vibe" than it's a lost cause unless you do a singaround where everyone accompanies themselves. Then, to me that's not a jam session but a mini-concert which is cool too.

Frank Hamilton