I'm not sure how long users have been able to opt in for the alpha build of Scrolls, Mojang's upcoming collectible card game, but you totally can. On the official site, simply scroll to the bottom and sign up for the newsletter. There, you can specify if alpha testing is something you'd be up for.
This wasn't offered when I had originally signed up my email during the reveal for Scrolls, so presumably I am not alone here, and this post will be of some use. If not, well, enjoy this video on the off chance that it's new to you.
Thank Buddyp450 for the heads up.