Jams are the time to make mistakes. In a Jam you can experiment, screw up, fall down, drop your axe and otherwise make a fool of yourself. The flip side of this is that you have to be tolerant of others who are screwing up. In any case good manners should be as important here as in any social situation. Rule 3 .
Jams are for beginners as well as experts.
Must know the changes. If you are an expert, teach the beginners, don't condemn them. If you are a beginner, it may be better not to play anything if you are not sure of the key or the changes. Rule 4 . Everyone gets a chance to solo - if not in every song then in every set for at least one verse.