The town has to hold the group
Choose a festival where the town can hold the group: food, shade, and a place to sleep within walking distance. Arriving at the same base matters more than the name on the stage. If the stay is an hour away, the night becomes a transfer problem and you see the festival in pieces.
Book as soon as the date is out. Large groups lose the beds first. A confirmed base organizes the rest: what time you head out, where you eat, how you get back.