In most bands the guitar fills a rhythm role. Sometimes it's the only rhythm player, sometimes there are several. [1] You need to give people the downbeat, the upbeat, help communicate when to start and stop the moves (phrasing), and support the melody by playing chords that work well. That's a lot! Where to start?

First, it's probably helpful to listen to some people doing it. Unfortunately videos of contra dances tend to be pretty useless here. It's usually hard to hear the guitar by itself, you probably have tinny laptop speakers anyway, and you generally can't see what they're doing with their hands. So the next time you're at a dance sit one out and watch the guitar player. Find somewhere close enough that you can hear the guitar above the other instruments. Hear where they fit in with the band, get a sense for what this is supposed sound like. Try this with a few different guitar players, try to figure out what's different between them.

Most of the time you're not at a dance, though. What can you do to practice alone? First, you need to be comfortable playing the most common chords and quickly switching between them: D, G, A, E, C, Am, Em, Bm, F#m. You need to be able to switch chords without thinking about it, without them being a distraction, but they're not the important part, just something to get out of the way. Once you're comfortable enough, try playing along with recordings. Don't worry too much about playing the right chord, start off by trying the chords to see if there's ones that sound better than others. [2] Once you've found some that don't sound too bad against the recording, start focusing on fitting in rhythmically. That's the important part. What are they emphasizing? How do they break the tune into pieces, so people know when to stop swinging?

Playing by yourself is ok, but playing with other people is more fun. Find existing jams/sessions or invite some friends over and host one yourself. Contra-focused jams are somewhat rare, but lots of cities have some sort of weekly Irish/Scottish/Celtic session at a bar. Do the same thing you would do with recordings: listen, try to fit in, try to support the music. Except when you're playing with strangers and still new enough probably don't be very loud: you need to be able to hear them clearly over your playing, and if you play too loud you might annoy them.

Even better than just playing with other people, though, is playing with other people for dancing. Many contra dance communities have open bands where anyone can show up and play. Ask people at your dance if there's an open band, and if not ask the organizers if they would consider scheduling one. (And maybe pass them this post.) When you're playing in an open band, pay attention to the dancers: you're playing so they can have fun! Notice when the music really seems to be meshing with the dance and when it's not as good a fit. Pay attention to the overall sound of the band, what there's a lot of and what's missing. Normally I'd suggest trying to figure out what's missing and play that, because that's what would make the band sound the best, but you're trying to learn your instrument. So find someone who sounds good and is playing something similar to your instrument and sit near them. Watch them, listen to them, ask them questions between the tunes, figure out what they're doing and do that.

Your options for playing for dances are also somewhat wider than open bands. Many dances allow sit ins, not mic'd, behind the band. Ask an organizer or write to the band ahead of the time (don't put the band on the spot). You need to be less intrusive than at an open band, because giving people like you a chance to play and learn isn't the focus, but there's still a lot you can figure out by observing how the musicians interact and what they choose to play.

Overall, play a lot. Play in small groups with friends, in large groups at open bands, play for singing following someone else's chords. Get comfortable with your instrument and with how things are supposed to sound and the rest will follow.

[1] Specifically, it's the only rhythm player about 60% of the time, at 69 of the 117 gigs in this sample.

[2] In books like the Portland Collection there are chords written out, but if you're trying to fit in with existing music as an additional rhythm player that's probably not useful. Why? Because the other rhythm player(s) probably already has chords they like, not the ones in the book, and generally not the same each time through. So you need to learn how to go from hearing music to playing chords that will fit ok.