Google Play
The flagship: a closed track by Google's rule — 12 testers and 14 days before production.
Google requires new personal accounts to run closed testing: at least 12 testers holding the app for 14 days before publication. Doing it alone takes weeks — here testers show up within an hour.
The mechanics: you publish a campaign, receive applications with emails, add them to Google Play Console or share an opt-in link. The tester uses the app for two weeks, you verify participation — karma is only credited for the real thing.
the pain of everyone who gathered 12×14 by hand
Here, leaving a track costs −15 karma and a mark in history. A tester who spent a month building reputation won't quit out of laziness.
Participation is confirmed by cross-checking emails against the Google Play Console track, not by a checkbox. Fake tests don't pass.
The campaign counts exactly opted-in: the tester is in the track, uses the app and is visible in the console for all 14 days.
True — and the buffer must join the track on day one: Google counts 14 days per tester separately. The campaign gathers 12+3 from the start — one dropout doesn't break the "12 continuous" requirement.