Do guests have to register every single time they open the album?

No — it's a once-per-album thing. The first time, a guest enters their name and an email or phone number, ticks the consent box, and that links them to their private photo set. After that, opening the same album again doesn't mean redoing the whole registration dance; they're back to their own gallery. It's the kind of friction that matters at scale — you don't want eighty guests each fighting the same form, you want them through it once and done.