Had to do this once for a group of about Girl Guides and Brownies.
Started off by asking if anyone knew a folk song. When the answer came back 'No' I said, alright, I'll sing the first line, then anyone can sing the next line:-
Wallflowers, wallflowers growing up so high.......
Suddenly I had about 25 people including Akela who knew a folk song!
From there on I just talked and sang and by the end they realised they knew quite a lot of Folk Music. Needed over an hour though!

FC