I've fit four people and four bikes on/in my Toyota Camry. Two in the trunk, two on the rack.... I'm assuming you're going to be taking multiple cars and not two vans. See if you can fit some bikes in their trunk. Also I've led group rides and we have fit maybe 8-10 bikes in a truck bed. Just set then normally in the trunk bed, then have three or four with the front wheel hanging off the back tailgate area...

For our cycling club, we rent a trailer though and just put the bikes in there.