Slow... Glad to see you're thinking of getting a gathering going up in your neck of the woods.

It sure sounds like a beautiful area. If I lived closer I'd definitley come.
If you don't mind my 2 cents.. here are a few thoughts for you.
I have been to alot of gatherings at state parks and have found that in general most folks coming to the gatherings in these parks don't mind if there isn't electricity. Many of them are equipped with a 12 volt system or simply do without and don't seem to mind. Decent toilets and showers however are a different story. Most people seem to prefer these facilities over electricity.
If you are thinking about checking out a state park, you can bring up the general info on the campground facilities in the park on the state park website as well as a map of the camp site loops.
http://www.allstays.com/Campgrounds/Wyoming-state-parks-campgrounds.htm This will tell you if there is electricity and what the shower/toilet facilities consist of.
If any of them are close to you in your travels you could also take a drive through to see what the sites/facilities are like. Are they wooded sites, no shade, level, etc. It will give you an idea of what might work well.
Another important thing to take note of is whether or not the sites at a state campground are reservable. If a particular park operates the campground on a first come basis it might be tough to plan for folks to gather there if they don't know for sure whether or not they can get a site.
I was looking at some of the info on Sinks Canyon and it looks very nice but it looks like they don't take reservations at this campground. Something to consider.
http://sinkscanyonstatepark.org/index.php?option=com_content&view=article&id=95&Itemid=78
As Grant mentioned, sometimes the state parks aren't too thrilled to hear that a group is looking to go there. It sometimes works out better not to tell them. Let the folks coming to the gathering know what park it's at and pick an area that might be ideal for a group to reserve sites in. EX: Gathering at Nice State Park - reserving sites in Loop A -sites 1 through 20. They can all make their reservations individually and the park doesn't know that a group is forming. What they don't know won't hurt them!
To add to the other suggestions about private campgrounds-If you are thinking about private campgrounds as a possibility, you could also check to see if they have a "group area" and how many small trailers or tents it will accomodate. Sometimes these work out well for a gathering because you can get a bunch of people all in the same area and they are slightly away from the regular sites. Some private campgrounds also offer discounts when there are a certain number of reservations made for a given group.
Whatever you decide do have fun with it. Don't make yourself crazy trying to come up with activities or places for folks to go. Half the time everyone's too busy chatting with everyone else and checking out each other's trailers that other things aren't necessary. Just pick a spot and go camping with your friends. The rest will fall into place while you're there.
