MrMustard,
Just from what I saw these 2 days, there were a number of random people that were parked there, and I think as long as you respect their wishes and not park on the right hand side, you are OK to park on the hill that leads into the campground.
To go further into the campground (left or right at the bottom of the hill), it seems that you need to be registered with them, per the signs that were up.