Every jam I have been in has been warm, inviting and welcoming. Hang out, listen to music, get a feel for the room. If you just want to sit there with your bass in your lap, no one will bat an eye, though they might try to gently nudge you into playing.
As for worrying about playing in front of other people, I will refer you to my Drool story. Until you can top that, you’re doing fine 
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