Might also try getting there before it starts so you can grab whoever looks like the leader and find out the sign up protocall. One jam I went to never had a signup, if the leader recognized you he'd toss you up-or if one of the other musicians knew you they'd say something. Otherwise you had to go ask to play on a break and they'd fit you in.....usually towards the end of the night.
Another one had a signup sheet, but for some reason it was on stage under the singers feet which made it a little awkward to grab.....
Make friends with the house bass player on a break, lots of times they'd rather sit and relax than play another 1/4/5 so if they find out you play bass they'll toss you up.
I'm with the poster that said they get handed a bass and the bassist dissappears...... has happened to me a few times

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