I would decline. There's no way in heck I'm shelling out $$$ to stay in a dorm (!) get up at 5am, and then be stuck sitting around in a remote location for six hours while the bride and groom do God-knows-what.
I'd decline too. My guess is that this is some kind of resort or retreat, perhaps religious or spiritual in nature. It's quite possible that the couple have enjoyed staying there, have friends that they met there, and see staying in dormitories and getting up before sunrise as part of a pleasurable experience. There are probably activities during the day, so that the couple and their friends think that the guests would have plenty to do.
But I have to say, any place that charges lots of money for a dorm bed, and regards having a ceremony at Oh-Gawdawful in the morning as normal, doesn't sound like it would have activities that would interest me.
I really want to hear an update, if only to find out what kind of place this is.