Camping Stadspark has let us know that because of smartly placing tents and campers they have made room for a limited number of tents; however, reservations are no longer possible.
If you take a bit of risk, you can try to place your tent at Camping Stadspark when you arrive on Saturday. They do not take reservations and the places are without electricity, but there are still a few small places for tents available.
If you want more certainty: Camping Groningen International is less than 10km from the festival and has plenty of places that can be reserved. Are you coming with a camper? Take a look at Camperplaats Kardinge, where you only pay tourist taxes!