You have to purchase a party ticket to stay in the park after 7:00. If you don't have a wrist band (which you get once you enter the park with your party ticket) you will not be allowed to stay at the park.
If you purchase a party ticket, it is a limited number of tickets, so wait times can be less depending on how many tickets have been sold. We have done the party twice and have found wait times to be less than no part days.