Assuming you are camping in the same campground, (like The Roo GA-laxy) 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 heading to your camping accommodation. Then just let the campground staff know that you’re with each other, and they’ll try to keep you together.
If you want to ensure that you and your friends are able to camp together no matter when you arrive at Bonnaroo, check out Groop Camping!