In the seven days prior to the grand opening, there are showings tentatively scheduled for four of those nights. (No, I don't know which four.) They are intended as press events, annual pass previews, and the like. Unknown if other guests will be allowed in, or even if the showings will take place during hours when others would still be in the park.
Since the show has already been extensively rehearsed during non-park hours, the show aspects need less rehearsal than new attractions typically do. There still remains working out issues with crowd flow and such, but those will be worked out over the first weeks the show is fully open - a few preview showings won't be enough to get all that worked out.