Susan
That's fine if you are the one 'running' the session - then of course you can put people straight as to the way you prefer things to go. What is difficult is if you're not a regular.
How right you are about welcoming others and esp. inexperienced singers/players, and explaining how it works. My problem is with the people who exactly don't do that, and think the audience should be honoured to hear them even though they ignore said audience and even insult them(and their fellow participants) by NOT HAVING LEARNED THEIR WORDS AND HAVING A CRIB SHEET!!!