Don T: "I have long advocated that joining in with a singer who is out front, concert style, should be by his/her invitation only, except for choruses.

Singarounds are somewhat different, and I tend to join in with others who I KNOW will not mind.

Sessions are by definition joining in events, and anybody wishing to perform without others' input should so indicate at the start of his/her song."

I think your brief summary here, makes a good deal of sense Don.