One way to actually reserve camping in JTree, if you have a group
One of my complaints about JTree camping is usually that the sites all fill up so quickly and it's hard to be guaranteed a spot. That's what nicer about the group camp sites! Sheep Pass campground is one of three group sites in JTree (along with Cottonwood and Indian Cove), all of which can be reserved online ahead of time. You just have to have a group with a minimum of 10 people.
This campground has 6 individual sites, and host large groups pretty comfortably. It's great for our group climbing trips, when we actually make plans far enough in advance. See here for all rock climbing info for the immediate area.
FYI, this campground gets its name because there actually can be big horn sheep spotted in JTree! Though we have yet to see any on our trips...
As with all campgrounds in JTree, remember that there is no potable water available here. For large groups, that means you need to be packing A LOT of water for drinking/cooking for the duration of your stay.