Also, what about asking people what they'd like to hear, then asking them to sing a verse with you, to teach you the song. They'd get a bang out of that — even if they can't remember all the words. And once you have the first line or so you can probably find the song for next time.

It's also lovely to make someone a star among their peers, which will be the effect if you call them up to sing with you.

This YouTube video, Black music from the 1940s might give you some ideas too.