Complex sentence: The site subsequently had a page refresher and those who could get into Pottermore were informed of "The Magical Quill" challenge, which would allow users who completed the challenge to complete the "early" registration for the site.
Split into simpler sentences: The site subsequently had a page refresher and those who could get into Pottermore were informed of The Magical Quill challenge. This allowed users who completed the challenge to complete the early registration for the site.