You can get in line right before closing, but expect to be escorted/guided out shortly after getting off the ride on the party nights. At EMH you technically could just wander the park and look in the stores and no one would know you weren't an on site guest unless you tried to get in line for another attraction and they tried to scan your magic band. But at parties, guests that are in attendance will have arm bands that are clearly visible to CMs. They will know you're not a part of the party and will be escorted to the front of the park, same as any dining guests who finish their dinners after the party starts and are not attending the party.