I looked at this, and thought maybe that campground is for groups, but no, it is individual sites. At first, I thought it was a private campground, but no, it's Forest Service. I do notice, however, that this campground has vault toilets, which make it not so attractive for some folks (they prefer running water). Let me see if all the sites are reserved. It's possible the group reserved 15 sites or so. If they were at one site, LE would have been called by other campers. There's a curfew and standard camping protocols that pretty much everyone adheres to.
Oh, there's no water currently at this campground. That would definitely make it unattractive.
For photos and other info, see:
Explore Prosser Family in Tahoe National Forest, California with Recreation.gov. Prosser Family Campground is located on the picturesque Prosser Reservoir within the Tahoe National Forest, 10 minutes north of Truckee. Visitors enjoy the area
www.recreation.gov