Two minutes to set up, then about a minute a day for a fortnight.
Open that on your phone and tap Become a tester. A Play Store link appears straight after — install from there, exactly like any other app. No security warnings, no "unknown source", nothing to work around.
If you already have the older version — the one installed from a file rather than the Play Store — uninstall it first. The two are signed differently, so the Play Store version won't install over the top and you'll get an error. Your account, friends and streak all survive; you'll just sign in again.
This is the real favour, and it's the reason the app can't be published without you.
Google won't let a new app onto the Play Store until 12 people have stayed in the test for 14 days without interruption. Not 12 downloads — 12 people who join and stay. If the number drops below 12 at any point, the fortnight restarts from zero.
And please play it every day while you're in the test. It takes about a minute — write your pair, guess your friends'. Two Truths is a daily game, so a fortnight of real daily use is the only thing that properly tests it, and Google looks at whether testers actually used the app rather than just installing it.
Turn on the daily reminder in the Stats tab and it'll nudge you.
The testing notes page covers signing in, adding friends, what's worth reporting, and what's changed in each version.
If the link says "App not available", it's almost always the wrong Google account — check the profile picture in the Play Store app and send Paul that address.
Anything else: twotruths@ampora.com.au, or just message Paul.