Complex sentence: On 10 August 2011, Pottermore sent a congratulations email to registered users, confirming that they would get early entry and that the beta period would begin once the sign-in button had reappeared on the website.
Split into simpler sentences: On 10 August 2011, Pottermore sent a congratulatory email to registered users, confirming early entry. This also signified that the beta period would begin once the sign-in button reappeared on the website.