Expect ANYTHING and EVERYTHING!
It's not gonna be all laid back 12 bars trust me, so kinda let the host know what your comfort level is. I literally started playing bass live on Beale St. here in Memphis without ever playing bass in a band before. I played guitar at many a jam just not bass.
For one thing know the number system
One, Four, Five etc: and be able to do that in keys you may not have played that tune in before. My band does Stevie Rays "Pride and Joy" in the Key of A, last jam the singer called it C. A short
four means it returns to the
four twice before the
five turnaround
(Sweet Home Chicago). Usually a good jam stays in the
I IV V most of the time but be aware that some songs vary that, the tune
"Key to the Highway" for example goes I V IV I, and many times you will have a singer "vamp" on the
ONE, around here they usually hold up one finger letting you know to hold and groove on the root note. Finally be able to know and play in minor or major keys. It matters not what kind of bass you play, so don't think you need a P bass with flats, and if you get lost just
"soft pedal" or back off a bit until you catch up.
I've done many of these and am going to one this week to audition a new guitar player singer LIVE at the jam ... so we are both under some pressure he has to give me what he wants on the fly, and my drummer and I gotta play it!
But hey have a blast it really is a great way to get into playing live and networking ... I met Brian a drummer at a blues jam and he is now our backup drummer when and if our regular drummer can't make a gig, and he is a TB member
(plays bass too)
Me at the Memphis Blues Society Jam as guest house bassist last Summer.

... Your gonna be fine, wish I could be there my family lives in Wentzville