I go along to a local jam session and it's always good fun. The session I go to, if there's going to be any covers then they're announced in the facebook group so those who don't know it can prepare. I always just learn the chord progression and go with the flow on the night, no one wants or expects a note for note cover.
Because I'm a bit crap, I tend to end up playing when the really good guitarists play, there's so much going on that they appreciate simple bass.
I always find everyone friendly and forgiving of my incompetence and encouraging. It's definitely improved my playing.