There is a group that shares sites on private property, they want you to volunteer to do the same on your property but it isn't a requirement. I actually joined and altho we own some acreage it seemed impractical to open it to any one to camp on. Especially considering the popularity of where we live. But I don't recall the name of the group, they send me occasional email but I could not find it searching my email trash not recalling their exact name.
Ah ha! I spoke too soon and found it with a quick Google search it is
this site--Boondockers Welcome