This campground didn't look like much from the road, but it is quite nice once you get in your spot. The website has little to no good information, so I hope to provide that here and help other campers.
Location: The campsite 10–15 minutes from Gardiner, a little town with a grocery store and a few restaurants. We stayed in a cabin in Gardiner most nights of our trip and camped only one night, just for the heck of it. Elk are numerous around the Mammoth area (the lodge and hot springs as well as the campsite). I got up at 1 AM to use the restroom and came back to find a female elk feeding 10 feet from my tent. She just stared at me, so I broke the 25-foot rule (the distance you are supposed to keep from elk and bison) to scuttle back into my tent. At around 5 AM later that morning, you could hear hooves walking around outside. Another tent camper warned us that they awoke that morning to an elk teething at the tent (not biting it to try to tear it, just checking it out). So if being up close and personal to horse-sized wild animals bothers you, skip the camping or bring a camper.
Campsites: There are about 80 campsites at this location. Tent-only sites are available, and among those some sites are reserved for backpackers/through-hikers. We were in tents and used two tent-only sites for our group of 10. Six people are allowed per site. If you are camping in tents, the managers when we went (June 2018) didn't care how many tents you had per site, and you can set up tents off of the tent pads (as a couple of our people did). The managers also didn't need to know specifically how many people were at each site, as long as each site didn't have more than 6. A ~3'x4'x3' bear locker is provided for each site, along with a picnic table and a fire ring. I liked the layout of both of our sties, as the tent pads were a good distance from the fire ring. We didn't have any trouble getting ultralight stakes in the tent pad ground at our sites (75 and 76), as other reviewers noted.
Facilities: As other reviewers have noted, there are bathroom and sink facilities, but no shower facilities at the site. We did not use the nearby showers at Mammoth Lodge. The bathrooms were clean and there were flush toilets at the entrance to the campsites and right by the tent-only sites. It was a little walk uphill to the bathroom from my site. No dishwashing is allowed at water spigots, bring a dishtub.
Check in: We showed up at 7:15 AM ish on the day that we planned to camp, and parked behind 2 other vehicles that were already queued up. By 7:45 three more were lined up. The campsite managers start taking reservations at 8:00 AM. The managers told us that they hoped to have all of the sites filled up by noon, so I think they go fast.
They track everything on paper, from what I could tell, no electronic system. There is no self-serve board or anything like that, you must go through the site managers to get your spot reserved. Only 3 of us went to reserve the site but our total group was 10 (which we told the managers) and that didn't cause a problem. To register, we had to give the driver's license number of the person reserving the site and the license plate of the vehicle that would be parked at the site. (So we had to go back to Gardiner to get a license plate number to complete our reservation, since we didn't know for sure when we would be able to return to the site.) We paid in cash and the manager said that was preferred. It was $20 per site when we went.
Management: The campsite is run by the National Parks Service, not Xanterra. The site managers stay at the site all season long and go around the sites on a golf cart toward the beginning of quiet hours to check that everything is in order. We did not make it to the site to finish setting up tents and settle in until 10:00 PM (the beginning of quiet hours) but did our best to be quiet and respectful, and it wasn't a problem, though one of the managers did come around and ask (rather sassily) why nothing was set up yet.