The only formal jams I have been to have been in Glen Rose. It’s a big campground and the circles pop up all over. Most (maybe 60%) prefer to stand so you can kind of work in or out of the circle. There are different skill set circles, but there aren’t signs or anything. If you walk up and they are playing Gold Rush at 260 BPM with the mando, guitar and upright playing synchronized harmonic parts, it’s probably not a beginner circle. You just kind of listen to see what they are doing. Typically there will be slower and faster groups… some that focus on gospel, others on fiddle tunes. I will say this. I have been a group where everyone (except myself) was a killer player, and we had some near absolute beginners come join in, and they were fine. As long as you pay attention to the volume levels and don’t hurt the overall sound, my guess is no one cares how great (or limited) your abilities may be. I had just picked up the mando, and they got the idea after a while when I kept passing on breaks. I had a hard time getting a nod once I switched to guitar, but all ended well.
My understanding is that at some places it is a bit more formalized, with “slow jam” circles and “expert” circles and such. I haven’t personally run into that.