Greg, those of us who are still in the folk scene are the ones who DID join in and DID sing along to the choruses even when we didn't really know them... No doubt you, like me, are both confident enough and good enough to not be intimidated - my point is, shrinking violets (as a lot of teenagers are) will be put off - it's just the same as if you went to a pub with a bunch of people you don't really know and they talked about something that you knew nothing about all evening to the exclusion of all else.
I personally never felt excluded or intimidated because I was always confident in the company of adults. Not all people are. A second factor is that listening to other people joining in on choruses you don't know yourself can also be pretty boring... I've seen a lot of young people who like to perform themselves but don't really enjoy listening to other people - sad but true.
But, that said, there are enough of us to keep things going!
Daisy