Been there last year and the year before.
Party guests will start to show up in their costumes starting at 4 (in fact, I'd wager that most party guests are in the park by 6), so if you want a taste of the party atmosphere, hang out then. Just be aware that the 3 overlap hours get very crowded, so don't plan on doing any e-tickets as walk-ons.
After 7, and continuing to around 9, expect to see picket lines of cast members forming up across the major walkways of Frontierland, Tomorrowland, and Adventureland, asking all guests to display their party armbands. Anyone caught without them are "encouraged" to leave the park. Band checks at the entrance to all attractions continues throughout the party.