The Robb State Park is very convenient if folks come from all over, as many folks are still working half to full days before heading out. You can get the group site reserved for what is a pretty good price per head, especially when the overall trip cost is considered. There are both fast food and quality restaurants, big grocery stores, raft/shuttle biz, liquor stores, etc right there too. It is a very good place to gather a group. Highland is good too, just a little more driving, but only like 5 extra minutes each way.
Rabbit Valley is >10 miles past the Ruby put-in, but more unpredictable. There are suitable sites very close to I-70, but its harder to find your people.
You are camping no matter what due to no camping policy at the Loma Ramp, and there are limited options within a <15 min drive. It is a relatively high density population with limited public lands/free camping close by.