Use your ears and common sense.
In a new session I can spend ages listening to what other people are doing to get some idea of what sort of session it is and agonizing about what to play if I want to start something.

Join in with tunes you know, and when you feel you've got the measure of the session have a go at starting something off if you feel confident it will fit in.

Nobody minds very much if anyone just joins in unobtrusively with what what others are doing.

With your bass, judging by my experience with a cello, you can join in with almost anything and it will add lots of musical value if you know or can guess enough of the harmony.

My feeling is that if you've the sensitivity to bother to ask here, you won't have a problem. Start quiet and build up slowly if you're not sure.

Oh, and if there's someone to ask before the session starts, why not? It can't do any harm and it's the beginning of making friends with some people.

Anahata