We didn't get the invite but we were staying at the Poly so we asked if there was a standby line (there was).
They are showing it for the next couple of nights. Our experience was we showed up at 7:15 and checked in with a standby line. DVC (of whomever organizes the event) know if there are a lot of no-shows by 7:30 and since they don't significantly overbook we got in with no problems.
We had a great time and there were plenty of open seats! Free popcorn and some Moana swag (kids activity books, etc).
One deal breaker for some people was that you are asked to check in your phones. One couple was quite upset as they had to have their phone for whatever reason so they left in quite a huff.
Anyways, great movie, fantastic setting for the movie (a movie with Pacific island characters shown at the Polynesian... Perfect!) and the kicker was that holiday wishes happened 10 minutes after the movie finished and they were beautiful.