There will be full backline for the jams....I played at Jemfest 2008, so I can tell you exactly how it works....
The scheduled performers go on during their time slots in quick rotation, so there is only about 15 minutes between each musician/band (if that!). Especially if it's just one guitarist playing to some backing tracks, then tear down is just unplugging your pedal board & cable from the amp. Some guitarists use a whole backing band, but still only 15/20 minutes in between acts to keep things moving along quickly.
As soon as the last performer is finished, they have the raffle for the guitars, amps, and other guitar gear (in 2008 they raffled off a Jem axe, a Carvin amp, and a bunch of other goodies).
The Jam starts, and depending on how many people are in attendance, musicians can just go up there and play, up to 3 guitarists at a time. There is only 1 drum kit set up and 1 (maybe 2?) bass rigs set up, but the whole stage set up is definitely oriented towards guitarists. You have your pick and choose as to which amp/cab you want to plug into (there are 3 different guitar rigs), and you can bring pedals or pedal board if you wish.
However, there is no need to bring your amp for the stage jam, as there will be no time to set it up (THERE ARE 3 AMPS/CABS SET UP ON STAGE)....the playing starts immediately, and it's basically plug and play.....The sound quality is very good, top quality gear is used, and you don't need anything except your guitar.
Even if you don't have a guitar, there are TONS there to use to jam with, but I know that people want to bring their own axe, so what I would recommend is to bring:
1) your guitar/bass
2) cable (although not necessary, but sometimes good to have as backup or if you have pedal board and need more cables/connectors)
3) pedals/pedalboard (but not required)
**Like I said before, even if you didn't have any of the above items, there are guitars/basses, cables & pedals available to use for the jams, so if you bring anything, it's for personal preference only. If you do decide to bring your amps, it would be for the hotel room jams only, but that's another story.... as far as the STAGE jams, if you are not a scheduled performer in the set list, you will not be allowed to set up your own amps/cabs. There are plenty up there all ready to go!!!!!!
If you have any more q's post reply here or pm me!!