PeerPlay
To add testers in Google Play Console, open your app's closed testing track, go to the Testers tab, add an email list or a Google Group, and save. Then share the track's opt-in link. A tester only counts once they open that link and accept it with the Google account you added.
This guide walks through each step for a closed test, which is the track that counts toward the 12 testers for 14 days requirement on new personal developer accounts. The menu names below match the current Play Console as far as I know; older layouts use slightly different labels for the same pages.
Step 1: open the closed testing track
Select your app in Play Console. In the left menu, open Test and release, then Testing, then Closed testing. In older layouts the same page is under Release, Testing.
You'll usually see a default track called Closed testing - Alpha. Click Manage track on it. You can create more closed tracks later, but one is enough for the requirement.
Don't use Internal testing for this. Internal testing is handy for quick checks with your own team, but the requirement for new personal accounts is about a closed test.
Step 2: add testers on the Testers tab
On the track page, open the Testers tab. You can choose who gets access in two ways: email lists or Google Groups.
With an email list, click Create email list, give it a name and add the testers' Gmail addresses, separated by commas, or upload them as a CSV file. Email lists belong to your developer account, so you can reuse a list for other apps later.
With a Google Group, type the group's email address, for example yourapp-testers@googlegroups.com. Anyone who joins that group gets access, so you don't have to add people one by one. The group must allow people to join, otherwise testers get stuck before they ever reach your app.
Fill in the feedback URL or email address so testers know where to report problems, then click Save changes.
Step 3: make sure the track has a release and countries
Testers can only install something if the track has a release. If you haven't done it yet, create a release on the same track, upload your app bundle and roll it out.
Also check the Countries / regions tab of the track. A tester in a country you haven't selected can accept the invite and still be told the app isn't available.
Then send the changes for review from Publishing overview, if Play Console asks you to. Google doesn't promise a fixed review time. It can be hours or a few days, and new apps often take longer. Testers can't install the app until the release has passed review.
Step 4: share the opt-in link
On the Testers tab, under How testers join your test, you'll find the link to join on the web. Copy it and send it to your testers, together with one line telling them to open it while signed in with the Google account you added.
On that page the tester taps Become a tester, then installs the app from the Google Play link on the same page. Closed test apps don't show up in Play Store search, so this link is the only way in.
If you use a Google Group, send the group link first and the opt-in link second. A tester who skips the group gets a not found or not available message.
Step 5: check that your testers count
Adding an address is not the same as having a tester. Someone counts toward the 12 only after accepting the opt-in link with the same Google account you added. Open your app's Dashboard to see the closed testing progress, with how many testers are opted in and how many days have passed.
If the number is lower than the people you invited, ask who hasn't accepted yet and whether they used a different Google account. Keep a few more than 12, because one drop below 12 can cost you days.
If you don't have 12 people who will stay for 14 days, PeerPlay is a free way to find them: Android developers test each other's apps every day through your real closed track, so they join with your opt-in link like any other tester.
FAQ
Where do I add testers in Google Play Console?
Open Test and release, Testing, Closed testing, click Manage track on your closed track and use the Testers tab to add an email list or a Google Group.
Should I use an email list or a Google Group?
An email list gives you control over each address. A Google Group lets people add themselves by joining the group, which is easier when you recruit many testers.
Why don't my testers show as opted in?
They have to open the opt-in link and tap Become a tester with the same Google account you added. Being on the list alone doesn't count.
Do internal testers count toward the 12 testers?
No. The requirement for new personal accounts is about a closed test, so add your testers to a closed testing track.