First of all, congradulations on finding people to play with! That is a real advantage that seems to be a lot harder to do than it used to be. Seriously... way to go!

Hands down, after persistant practice, playing with others is the best thing you can do to make your music grow. Jamming is a surprising endevor and that is what makes it so powerful. Once you are comfortable with the group, your own playing will really excellerate. I don't know if it is from being streached by exposure to more advanced playing, or excitement at participating in making music, or losing yourself in the moment...but Jamming with others really super charged my learning process.

Do they use lead sheets (printed words and chord names)? If so get copies so you can get up to speed with the chord changes on your own time. Get a tape recorder and record the group! Pay close attention to the rhythm as you play, that is critical to making music not noise. LISTEN, LISTEN, LISTEN!! You are trying to blend with the sound the group makes. Most important.... Have fun!! This is where REAL MUSIC HAPPENS! ENJOY THE RIDE!!