PeerPlay
If you are on a new personal Google Play developer account, you need at least 12 testers opted in to your closed test for 14 days in a row before you can apply for production. Friends run out fast. Strangers have no reason to help.
Other developers in the same spot do have a reason. They need 12 testers too. That is the whole idea behind a tester swap: you test their app, they test yours. It is how I find developers for PeerPlay by hand, one at a time, and the difference between a swap that works and one that is ignored is almost always in how you ask.
Where to find developers who need a swap
The best people to ask are the ones who said, in public and recently, that they need testers. They have the problem right now, so your offer is not spam, it is an answer.
I look for these posts on itch.io and in developer forums, and reply to them personally. Subreddits and Discord servers about Android development have the same kind of posts. Sort by new, because a developer who posted today is still looking, and one who posted a month ago probably finished or gave up.
Skip giant "drop your link here" threads. Everyone posts, almost nobody installs, and you end up with testers who opt in once and never open your app again.
What to write
Keep the first message short and specific. It should say what you offer, what you ask for and what makes you reliable. Something like this works:
"Hi, I saw you need testers for your closed test. I'm in the same spot with my app, a [one line about it]. I'd like to swap: I'll join your test today and use your app every day for the full 14 days, and you do the same for mine. Here's my opt-in link: [link]. If you're in, send me yours."
Three things make this message work. You name their problem, so they know it is not a copy-paste blast. You promise the full 14 days, not "I'll install it", because they know installs alone do not get them through. And you give your link right away, so saying yes costs them one tap.
Go first
The most effective thing you can do is join their test before they join yours. Opt in, install the app, use it for a few minutes and send one honest line of feedback. Then remind them about your link.
This turns a request into a debt. Most developers who have just received real feedback on their own app will return the favor. It also filters out the people who would have dropped anyway: if someone ignores a swap where you already did your part, they would not have lasted 14 days on your test either.
Keep your side of the deal
A swap only works if both sides stay for the whole 14 days. If you uninstall their app on day 6, you may push them below 12 and undo the whole point of the swap. They are counting on you the same way you are counting on them.
Open their app every day and actually use it. Set a daily reminder if you need one. Leave feedback when you find something: a crash, a confusing screen, a typo. It costs you a minute and it is the most useful thing a developer gets from a closed test.
Keep a short list of who you swapped with, which app, and the day each test started. When you are juggling five or six swaps, it is easy to forget one, and the developer on the other end will notice before you do.
When swaps stop scaling
One swap gets you one tester. To reach 12 with a buffer, you need around 15 swaps, which means testing 15 apps every day for two weeks. That is a lot of phone time, and the more swaps you run, the harder it gets to keep track of who is still active on your test.
That is the problem PeerPlay was built for. It is the same swap, organized: developers test each other's apps in tracked 14-day rounds, the app checks that each tester actually opened your app for a real minimum session each day, and testers who go quiet are removed from the round. Whether you swap by hand or through an app, the rules are the same: ask the right people, go first and stay for all 14 days.
FAQ
Is swapping testers with other developers allowed by Google Play?
Google asks for real testers who use your app during closed testing. Developers testing each other's apps are real users with real devices. What matters is that testers actually use the app, not who they are.
How many swaps do I need for 12 testers?
Plan for more than 12. Someone will be slow to join and someone will drop out, so a few extra swaps protect your count.
What if the other developer stops testing my app?
Check your opted-in count in Play Console every day. If someone drops, send one polite reminder and start a new swap right away instead of waiting.
Should I swap with someone whose test already started?
Yes. Your 14 days and theirs do not have to line up. Just keep using their app until their test is done, even if yours finishes first.