i'm going to a tavern tonight with my drummer buddy and another friend who plays bass but isn't that great.
but what i'm wondering is, how do public jams like that work?
it said rock, blues, country. any instrument pretty much
when i've jammed with my friends i usually came up with something and they just latched on.
but this is open to anybody so i'm wondering how this would work?
any insite?

Yeah, I don't see much difference between tonight and when you and your friends normally jam.
live to play live
hmm... If your just gonna improv jam then just tell 'em that you will play something simple and follow your lead. Or jam to something that ya'll have rehearse many times
I'd say you need to build a nice catalog of songs that you know and most likely what your buddies know. that way you guys have a base to start with. With my band we hashed out 30 covers for a New Years Eve multi-genre show we had over here. the sad thing was most of the crowd wanted hip/hop and dance music. how ****ing lame is that?
Just relax, and play simple progressions until everyone gets the vibe.
Let's say that I have this friend, who is certainly not me, and he would like to say that you should just relax and play as you would normally. Jamming with other people shouldn't be really different from jamming with new people, better yet, the latter should be more fun and more educational.
Just be sure of yourself (or tell your friends to be...) and try to be friendly. Don't act like you're the be-all-end-all, but don't be too humble either. Play, enjoy, and don't be afraid to think outside the box a bit.