I set up the first NW forums rally and others have set up several since. Pretty much you just pick a place and tell folks where you're going to be so they can make their own reservations if necessary and show up. You can also reserve a group site ahead of time and let people pay you back. That gets trickier because you need to know how many sites to reserve, and you may end up adding more later. Rangers have been pretty helpful when asked about organizing a group gathering - or you could do like we have and get your own personal Airstream park ranger to grease the wheels (right, Campcollector
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People talk about insurance, but I don't see why. Anyone CAN sue you for anything. But there's really no reason for anyone to be liable for what goes on. Everyone involved are adults, the forum is usually at a campground of some kind, I just can't see where personal liability plays into it.
Once you've picked a site and people are coming, then you can organize (or guide) activities, potlucks, whatever the group is into.
It's not that big a deal and it's fun. Think of it as going camping. You tell your friends where you're going to be. They all show up, and it's a party. Fun!