Assuming you have the same kind of car camping pass (like a GA car camping pass) you will be camped with the people that arrive at the same time as you. So if you want to camp with a few friend’s cars, be sure to arrive and get in line at the same time (we’d suggest meeting up about 40min away from bonnaroo, then driving together from there on).
Don't worry if you get a little separated in line or at security, you can regroup right after the security check before rolling into the campgrounds. Then just let the campground staff know that you’re with each other, and they’ll try to keep you together.