i have the same problem. I recived the invitation e-mail on
ricki1993.scorpione@hotmail.it but i accepted to be tester with
ricki1993.scorpione@gmail.com (it's right, it's my main account). If i access to google play with
ricki1993.scorpione@gmail.com it only say to unregister. I tried to add
ricki1993.scorpione@hotmail.it on google play but it only let me to register to the app.
EDIT: i resolved my situation. I figured out that if you become a test with an email/account different to one you recived the mail you cannot download. Simply open the mail from your browser, select download now. At this point you're on the page of my first screen (gwentbeta.PNG). On the top right select the right account (if it's not listed add it) and become the tester. then you can download. I hope this help someone