Do this AFTER joining the group — and note the Play page is in English only. Scroll to the bottom and tap the blue “Become a tester” button. That is the step that officially registers you.
After you tap it the page reloads and looks almost the same, so it’s easy to think nothing happened. The install link is the small blue “download it on Google Play” text in the second paragraph — tap it, then tap Install in the Play Store.
Tap the image to enlargeIn the second paragraph of the reloaded page — tap the small blue link circled in pink.
The store link is personal to your opt-in page, so there’s no button here. Still seeing “Become a tester”? The page hasn’t reloaded yet — reload it and look again.
Please stay opted in
Please stay in the test for at least two weeks, and don’t tap “leave the program”. Leaving early hurts the beta for everyone.
Seeing “app not available” in step 2?
You’re probably signed into a different Google account than your phone’s Play Store uses, or your group join hasn’t propagated yet. Wait a few minutes and retry with the same account.