This may or may not be helpful, but there is a global map of oldtime jams oldtime-central.com/jams/ and many of us play in both oldtime and bluegrass jams (a reason to acquire another banjo or two as if a reason were needed). I don't know what might be going on in the Black Hills, but take a look. Good luck!
Yes, I haven’t gotten around to it yet, but I plan on posting on mandolin cafe, and I need to find a guitar site that might be appropriate. We have recently moved here, and as far as I can tell are kind of in a music desert, there being only one music store I have found in the entire western half of the state. However I know there is a yearly festival south of Rapid City, and a very small festival in Buffalo Gap... so I know there are some players out there.
My thinking posting here, was that any banjo players I come across would be likely to know other musicians.
I have been to Hot Springs many times and plan to move there next year. Several years ago there was a music store downtown, owned by a wonderfully gifted guitar player. I had the privilege of participating in an impromptu jam session with her and the locals. Sad to say, she has passed away and the store closed. It may have reopened under another name and owner.
But, just so you know, there are some great pickers in that area.