What do you need to know?
How the jamming circle operates. Some you just sit down and start playing. Others you are invited in, or not. Sit with your instrument outside the circle and wait and watch for a nod to come in.
Here is what happens in Mountain View, Ark. My home town, Mineola, Texas has jamming down town and uses Mountain View as a guide, it is all acoustic every Saturday - during the summer months starting at 6:00 PM. During the rest of the year, 9:00 AM till......
Notice it could be all acoustic, no amps or drums allowed. Just something you need to check on before asking for an electric outlet.
The name of the song and the key will be announced. The chord progression is left to you. The type of songs played in my neck of the woods I get buy with faking a I-IV-V7 progression. If I do not know the song I watch the rhythm guitar's fretting hands and change chords when he does. Helps that I play rhythm guitar so I can recognize what chord he is fretting. Roots and root-fives will get you by, no need for more than this if you are not already there.
Some circles the caller that calls the song plays it from verse one to the ending and everyone else augments his efforts. When he finishes the next person in the circle calls a song and the key and then plays this song. Again everyone else augments his efforts with accompaniment chord tones, strummed or played one note at a time as we would do on the bass. Again root and or root-five is enough. This is how it is done, most of the time, in Mineola.
Other circles the caller will play a verse or two, perhaps the chorus and then pass the song to the next person in the circle who will then play and or sing a verse or two before passing it to the next person in the circle. Should you not want to accept this lead, just lean backwards and the songs will pass you by. This is how it is done, most of the time, in Mountain View.
While sitting outside the circle take mental notes on how this circle operates. I have never had anyone get upset with a newby if the newby was trying. If in doubt, just sit this one out.
You can practice jamming by calling up some backing tracks and playing along with them. So you miss the key, it's just you and the video and the video is not going to mind.
Go have fun.