it depends on the host(s) whether a 'jam' or an Irish session, is open to anyone who wants to play. In general that should be the case. However it is perfectly appropriate to mention to someone, (take them off to the side not embarrass them in front of everyone) if their playing is disturbing the others, ie. a guitarist who has no clue what chords to play, a percussion player who is too loud, or not in rhythm etc..
as it does ultimately spoil it for the others.

We got to our session once and found our host (who is blind) surrounded by 5 Bodhran players around the table, and the melody instruments had no place to sit. They didnt get it, they think that if they get there first they should be able to sit where they want, or for that matter whether theyre even welcome.

Session etiquette is common sense, but sometimes you must choose between being polite and having a good session.