I'm not a musician, but I'm married to one who has played a regular monthly contra-dance gig for several years. Here are a few things I've picked up from observation.

The group probably has a large collection of tapes. Borrow them in order to learn the tunes. Talk to the callers to find out what tunes are most in demand. If you read music, talk to the musicians who have played for the dancers before. They probably have a large collection of photocopied tunes. Borrow them and make your own copies.

Dancers are VERY forgiving of your musical shortcomings. They're usually so absorbed in their dancing they won't notice your mistakes. They're probably happy and excited to be able to dance to real musicians instead of tapes. Rhythm and endurance are more important than musical accuracy. Playing for dancers is a good way to get a LOT of practice. It's also a good way to get to sit in with more experienced musicians.

Pay attention to the caller, especially if he's teaching a new dance, or new dancers. He may ask you to stop and start a lot, or change the tempo.

It's a good idea to have someone who plays piano or electronic keyboard. They can set the rhythm by pounding out the bass notes. (Which reminds me: I've always thought "Boom Chicks" would be a great name for an all-female contra dance band. Any takers?)