As fineshot said more information would be needed.
You can almost certainly rule out UHF (frequencies above 300MHz) unless you see these guys running generators that can power a sufficient amp to get over those mountains.
From there I'd be guessing 2m as the most common (144-148MHz) if these are ham operators (or maybe 6m- around 50Mhz). If they are not hams it could be a shot in the dark. I'd bet that inter-park communications, even on the scale of a park the size of Yellowstone, might be tough on HF (3-30Mhz).
I'd look into some of the local ham clubs around Yellowstone to see if any mention the "Yellowstone Net" or anything. If you see a "wolf watcher" with a radio, politely ask him the model (write it down) and we can point you in the direction of that radios range. Another helpful tip might be pictures of radios (or antennas- which are sometimes more telling than the radios).