The term "jam band" revolves around iconic groups like The Grateful Dead and Phish. Dave Matthews, The Disco Biscuits and many other more modern bands have hit the road to perform music in a half-orchestrated, half-improvised format. The "half-orchestrated" part is the key to starting out as a guitarist in a jam band: Write songs. Don't stress over the jamming portion. You have to have a starting point. Most jam bands compose songs and then go from there.
Rehearse the songs repeatedly. Know them inside and out. Know the chord changes, the time signatures and the scales. As you rehearse, add variations to the song. Insert bridges. Let each member take turns soloing over the main riff. Practice jamming.
The most important step to remember while jamming is to not stop playing until the whole band stops. You can fade out and fade back in, briefly. But don't stop in the middle of a run because someone messed up, or because you hit a sour chord or dropped your pick. Keep going. If you drop your pick, strum with your fingers until you can find a new one.
In the jamming rehearsal, pick a clear stopping point. Any band can jam. But a really tight band will know when to stop on beat. Use signal riffs. Tell the band, "OK, when you hear me play this (...) we're going to stop at the end of the next measure."
Although you are practicing jamming, don't consider your practice riffs the ones you have to play. Jamming is supposed to include an element of improv. If you simply write a 15-minute song, you are not jamming in the "jam band" sense of the term. Keep writing to the first few minutes of the song and let the tune carry you to a designated end.