I know at AK, there was a enterance that you went through at the gate. They asked for your name, looked it up, and let you through. There was a big sign above that area.
Last year we had an 8:00am ADR and the park did not open until 9:00 (At H & V) CM's had a list at the front turnstiles of those with ADR's. As long as your name is on the list they will let you in. They also add several CM's posted down the main street and over toward H & V directing traffic and letting people know where they need to go (and making sure they didn't go where they weren't supposed to be)