call 303 823-6150, which is the number for Meadow Park on-site camp host. They should be able to tell you. If not call the parks and rec folks whose number is on the Lyons Chamber of Commerce page as well as other places.
My recent experience is that you should be able to set up early there. However, I had to pay $20 to stay in Meadow Park last week prior to the academy line. That's the amount per night. Cash. I assume you will have to pay as well
There were already two folks setting up under the trees and by the river in Meadow Par when I was there on Thursday, so I'd see if you can go earlier than Sunday
Go to Info/Guidelines and other sections of the RockyGrass website to read their rules - that's the officially definitive place, but the parks are run a little differently - call their number
Listening to:
the attic fan as it creates a breeze through my window