Beta Period was the testing period for the Pottermore site. One million people were granted early access to the site through a Magical Quill challenge from July 31 to August 6, 2011. Pottermore let small groups of users in to the site from August 10 to September 27, 2011. The Beta Period was supposed to end on October 31,2011, but was extended in October. The closed Beta Period finally ended on April 14, 2012, when the site was opened for public, though the feedback links still marked it as Beta.
Beta testers usually (but do not always) have lower numbers in their Pottermore Username.