Installing on iPhone

iPhones install through Apple's TestFlight (Apple's way of sharing family/beta apps).

Install TestFlight from the App Store.
Open the TestFlight invitation link from the Get-the-app page (or the link an admin sent you).
Install Our Family Forest from TestFlight, then open it and sign in.

If the iPhone button isn't live yet, the family's TestFlight is still being set up — check back, or ask the admin who runs the archive.