Three steps, about two minutes
Do them in this order. Step 2 will not work until step 1 has gone through, and the store page will show "not available" until step 2 has.
Join the tester group
A Google group. Click Join group - no posts, no mail. Use the same Google account your television signs in with.
Become a tester
Opt in on Google's own page. It answers "You are a tester" when it has taken - give it a minute before step 3.
Install it on the screen
On the television, open Google Play and search Mafia - or install from a browser and it arrives on the screen by itself.