PeerPlay
Most developers assume that adding an email address to the tester list makes that person a tester. It doesn't. Adding someone only gives them permission to join. They're counted as opted in after they open your opt-in link and accept the invite with that exact Google account.
That gap explains most "0 testers" cases on its own. The rest come from a handful of setup problems, and it's worth going through them in order, because the first ones are both the most common and the fastest to check.
1. Nobody has actually accepted the invite
Go to your closed testing track and open the Testers tab. You'll see your email list and the opt-in link. Copy that link and send it to every tester directly, even if you think Google already emailed them. Adding people to a list doesn't reliably send them anything, and a tester who never saw the link can't opt in.
Ask each tester to open the link on a device where they're signed in with the same Google account you added, and to tap the button to become a tester. Installing the app from the Play Store afterwards is a separate step. They should do both.
2. The Google account doesn't match
This one catches a lot of people. A tester gives you their work email, but their phone's Play Store runs on a personal Gmail. Or they have two accounts on the device and the browser picks the wrong one when they open the link.
The account on your tester list, the account that accepts the invite and the account that installs from the Play Store must all be the same. If a tester sees a message that the app or the test isn't available for their account, a mismatch is the first thing I'd suspect. Ask them which account is active in the Play Store app, then add that one to the list.
3. The release isn't live on the track yet
The opt-in link only works once there's a closed testing release that Google has reviewed and published. If your first release is still in review, or you saved it as a draft and never rolled it out, testers can open the link and hit a dead end.
Check the release status on the closed testing track and in Publishing overview. If changes are waiting to be sent for review, send them. Nothing about testers will start moving until the release is out.
4. The list isn't attached to this track
Email lists live at the account level, and each track chooses which lists it uses. It's easy to create a list, fill it with testers and never tick it on the closed testing track, or to tick it on internal testing by mistake. Internal testers don't count toward the closed testing requirement.
Open the Testers tab on the exact closed track you're running, confirm your list is selected and save. If you have more than one closed track, make sure the testers joined the one you're counting on. Also check the track's country availability; a tester in a country the track doesn't include won't be able to install the app.
5. The number just hasn't caught up
Play Console statistics aren't live. If testers joined in the last few hours, a zero may simply mean the count hasn't updated. I wouldn't start changing settings until at least a day has passed since the first testers confirmed they joined.
What you can check right away is the tester's side. If they open the opt-in link again and it tells them they're already a tester, they're in, and the number will follow.
Once the count moves
Getting off zero is the easy part. The 14 days only work if at least 12 testers stay opted in the whole time, so keep a few more than 12 and keep an eye on people who drift away. That's the part PeerPlay was built for: testers are tracked for real app sessions across the 14 days, and ones who go quiet are removed from the round instead of silently dragging your count down.
If you've gone through all five checks and still see zero after a couple of days, contact Google Play support from Play Console. At that point it's worth letting them look, rather than rebuilding the track and risking a fresh start.
FAQ
Do testers need to install the app to count as opted in?
Accepting the invite through the opt-in link is what makes someone opted in. For the closed testing requirement, though, you want testers who also install and use the app, since the production access application asks how testers engaged. Ask every tester to accept the invite and then install the app from the Play Store.
Can I use a Google Group instead of an email list?
Yes. A closed track can use a Google Group, and anyone who joins the group can then accept the invite through the opt-in link. It's handy for larger tester pools, but the same rule applies: the account in the group must be the one used in the Play Store on the tester's device.
Why can a tester open the opt-in link but not find the app?
Usually the tester is signed into a different Google account in the Play Store app, the release hasn't finished rolling out yet, or the tester's country isn't included in the track's availability. After accepting, the Play Store listing can also take some time to appear for them.
Does removing and re-adding testers reset the 14 days?
The requirement is 12 testers opted in continuously for 14 days, so anyone who leaves stops counting. If your count drops below 12 during the test, the continuous period is broken. Avoid removing testers mid-test just to fix a display problem; check the causes above first.