First you need Testflight, which is an app you can use to test apps in development, even before they’re in the App Store. Then you come back to the message that you replied to and click open the Avalon app in the TestFlight app to accept the invitation to test it. Then you can install it from TestFlight.

Also joined lemm.ee after lemmy.one went missing. Wish I’d made backups of the communities I was following but it’s going well so far.