Hey Blakeney, I can certainly sympathize with your anxiety about jamming for the first time with guitarists who will probably be doing alot of improvisation. Unless you've been practicing improvisation with riffs and licks, and know the notes and chords belonging to specific keys, you're not going to be able to do what they do. However, if you do go (even if you just sit in and ask questions the first time) I'm sure they will set you up with a few chords in the relavent key and have you just play those in an improvised manner.

Hanging and jamming with musicians more skilled than youself is a great way to supercharge your learning... a lot. Most guitarist want nothing more that to be able to share their guitar wisdom with others. They'll probably set upon you like a pack of musical hyenas in a teaching frenzy.

Do it! I very much doubt that you'll regret it.

Nicolai