I agree with what Stevemack said.
I've done the Jeepers Jamboree many times (I was even a cameraman for
Rick Russell's "Crawling the Rubicon" video #18), you are in for a hell of a good time!
As a reminder, you will be driving a difficult trail for pretty much an entire day or more, and then you will be tent-camping at the Rubicon Springs camping area. You will need to carry everything you need to camp (clothes, tent, sleeping bags, snacks, cooler, tools, etc.) - the Jamboree supplies excellent food, but you need to carry everything else. From the looks of your icon, you have a standard (short-wheelbase) Jeep. If you take four people, you will need to keep your back seat in, and find a way to carry the above gear for four people. Personally, I don't know three other people I like that much.
Most of the people I've seen on the Jamborees have just a driver and passenger, pull the rear seat like Stevemack above, and load all their gear in the back. That's what I would recommend, especially if it's your first time. Make sure everything is secure - you can see how Stevemack has everything strapped down.
I have an unlimited, and to make room for two 85lb German Shepherds, I loaded everything on two racks on the back.