Pick out the best guitarist to play lead and pick a great vocalist to back you up on the harmonies. If there's a harp player, percussionist, or drummer in the crowd, invite them to join the fun.
Choose a 'standard' that everyone will know.
This exercise will provide you with the following benefits:
- You'll make new friends
- The crowd is more likely to listen when they see a new configuration on stage
- If you're new to the open mic, the 'regulars' will be more accepting
- You'll have more fun
I've seen this in action for years. It's a winner!