Steeleye, our general style in a sing-along is for someone to "lead" a song. It's more or less considered "their" song, if they want the whole group to join in on the verses they may need to say so; otherwise, unless it's a very familiar song, the group will join on the chorus and let the leader sing the verses.

If you're hoping people will join in and/or jam on a song, just say so. It isn't necessary though.

I agree with Bill - the best thing to do at an event like the Getaway is to sing the songs you've been singing the longest and know the best. That way it all flows and people will pick up on the energy.