Possibly hack: Install the extension. Open it and click on the "sign up" button or whatever other thing is there that'll take you to the waiting list page. Then delete the extension. Alby will (at least as of a couple of months ago) ask you to fill out a form saying why you deleted the extension. I told them it was because I couldn't get an account, and less than 24 hours later, I got an email with an invite.

Might not work now, but it's worth a shot.