Watch the B Minor Blues - Part 4 online guitar lesson by Jeff Scheetz from Blues Jam Survival Guide
Don't forget as you are listening to the other players you are jamming with to notice what register they are playing in. In other words are they playing a lot of low notes? Or is their chord voicings all up really high on the fretboard? This will help you determine which chords will work best. If they are playing low - you go high, and vice versa. You really have to experiment, but usually you don't want to all be playing the same chords boring.