No, it's not ticketed. It's more like, it's not intended for the public, so they don't really cater to casual viewer, but they don't stop you from watching either. The proper seating is for invited guests, and it takes up the majority of the prime viewing area, but the public is allowed to stand behind and off to the side to watch, as long as they keep walkways clear.
Seating is limited to invited guests (there is roughly 1000 seats.) But there are benches available to first come first serve, and a lot of standing room. It really is a gorgeous affair. And it is easier to get spots for the second performance than the first.
We managed to squeeze in last year (our first time). Yes it was packed and yes it was a long time standing and yes it was an obstructed view, but we enjoyed it IMMENSELY and hope to go back this year. It was just beautiful. Great music and very.... Christmasy, for lack of a better word!