Learning is different from playing. Go to jams and learn songs even if you can’t play with the other musicians yet.
If you can’t keep up with the musicians, back out of the circle but sit where you can see someone who is playing your same instrument or playing a guitar. I think it’s easier to read chords off a guitar than off a banjo or mandolin.
Listen to the musicians. Sometimes they’ll tell the chords ahead of time. Write them down so you can practice at home. Sometimes they’ll just say what the weird chord is. Write that down and then listen and look to see when the regular chord changes come and when that odd chord shows up. Write down the chord progression to practice at home.
If you have some questions, wait until the song is done, then ask someone what the chords were. Most folks are very helpful.
Goal number one is to try to focus on the music instead of on your chords, your fingering, your strumming. That’s a tough goal, but too many beginners focus on how they play the chords or strum the rhythm instead of hearing how they play the song as a whole. The more you can hear going on within a song, the better player you will be.
To learn a break, a lead, to a song, I practice while playing backup. Instead of just strumming, I do a pick-strum and try to figure out the melody on the picked notes. Or else I strum but use different chord forms up and down the neck to try to get the melody to come out on the 1st string. If I have enough time to be successful, then when it’s my turn I know where enough melody notes are to make a decent stab at playing a lead to a brand-new song. If I don’t know the melody, then I shake my head so they bypass me.
The goal of a jam is to have fun playing music. It isn’t to show off how much better you are than everyone else. A jam session is a musical conversation. You don’t want to hang out with show-offs and braggarts so don’t do it and don’t be one.
One the other hand, when you appear at a jam for the first time, treat it as an already on-going conversation. Listen for a bit unless someone invites you to join in right away or you’ve heard that it is a wide-open jam. Listen for how you sound. Make it sound better, not worse. Be in tune. Don’t play too loud. If someone takes a break on a quiet instrument, quiet down yourself. Like a regular conversation, it all comes down to listening. If you can’t hear the other person, then how can you respond?
Usually, the person who starts a song is who ends it. He is also the person who signals the next person to take a break. If you shake off a break the first three songs, they probably won’t put you on the spot anymore. If later on you decide to take a break, you may have to speak up or step in. Usually, the breaks go around the circle as does the choice of songs. Sometimes a lead person or group of singers will do several songs in a row. That can appear rude (and may actually be) but it can also just be excitement and getting in a groove. Most people who appear rude are just excited. They love the music. Don’t get too upset and hope they won’t get too upset with you.