We're now learning how fans will be able to get into the beta version of Pottermore!
Details were revealed via the official Pottermore Insider blog. The gist: Between July 31st and August 6th, one new hint a day will be posted on the Pottermore website. You will use that hint to access a special website that will in turn allow you to sign up for the Pottermore beta. There are a limited number of beta invites each day.
If you do manage to get one of the daily invites, this will not grant you immediate access. You will have to wait "a few weeks" for your Owl.
Full details are available at this link.