If you're looking for a group ride the Thursday night 6.30pm ride is a lot of fun. Briana is also a great tour guide of the Fort.
As far as other info you might need .... there's a state park fee to enter. The trailhead is easy to find. Watch out for fresh horse bombs. There's a bike shop (Custer Cyclery) and a spot to grab a beer and a burger (Players?) about 3/4 mile away from the Fort.
It's my "home spot" to ride and even though I'm just a beginner, I do love it out there. I hope you enjoy it as much as I do. If you see blood stains on any rocks while riding, they are most likely mine. Wave and say hello.