iOS: Invite TestFlight testers with a public link.

Apple Beta Testing Made Easy, A big Thanks to TestFlight and Apple

Now you can invite anyone to beta test your app, by simply sharing a link. Share your link anywhere and it will lead to an invitation in the TestFlight app. You won't need to manage an email list and testers can remain anonymous.

To enable your link, you'll need to be an Admin or App Manager. Go to your app’s TestFlight page, click any external tester group, and click Enable Public Link. You'll have the option to set a limit for the number of testers that can join a group through your public link, and you can disable the link at any time.

Source:
TestFlight beta testing overview (iOS, tvOS, watchOS)