Some of you may or may not be aware that ArenaNet are going to be inviting people to the Heart of Thorns Beta soon.
As for how to get into the closed beta there is unfortunately no direct sign up. Invitations for the multiple beta events, as they occur in the near future, will be sent to any players signed-up to receive the Guild Wars 2 newsletter. While signing up for the newsletter does not guarantee you a spot, it is so far the only way to get an invitation.
You can sign up for the newsletter here.
And of course here is Wooden Potatoes video on the closed beta: