First of all you need to know what chords belong to which key, then most of the jam sessions will be with a 12 bar blues or rock chord progression, learn those to.
As for lead impro, make sure you can play pentatonic major and minor patterns and know with which key they belong.
95 % of the songs have one of the following chord progressions; learn to recognize them by ear;
I-IV-V ( 12 bar )
I- VIm - IV- V
I - IIIm- IV - V
Many bridges go up a whole tone to then go the the dom7 chord
example; I-IV-I-V then II (major ) and play some chords in the new key and last chord will be the IV 7 chord from the new key which happens to be the V7 chord of the original key.
So you see, without the theoretical knowledge you are not going to get to far.
One other tip; if you're not up for lead playing, there is no shame in telling that to the other people and just keep it with rhythm playing, there are more then enough people out there that just love to improvise for hours on lead instruments ( a little to much IMO )