It's open to anyone. I wouldn't really call it crowded though. The gate looked scary with all the kids arriving together on buses, but inside the park it isn't bad.
Chik fil a has the longest line today from what I've seen. Things are picking up a little, but Diamondback and Banshee were a station wait, Mystic was steps, and I rode on Vortex with only 5 other people on our train.
Vortex was amazing for this Arrow looper fan.