Benjamin Mayo, writing for 9to5Mac:
Apple is rolling out a new TestFlight feature which enables developers to share a public URL for an app beta. Customers can simply open the link on their iPhone or iPad and automatically enroll into the beta testing group through the TestFlight app.
This feature was announced back at WWDC in June but has only just started showing up for developers inside the App Store Connect interface. Previously to public links, developers had to manually ask people for email addresses and then send out invite in emails to each person individually.
As someone who's routinely testing dozens of apps for review purposes, this sounds a lot more convenient than the current email-based invitation system, both for developers and users. By default, developers don't see the names and emails of users who sign up with a link. I have a feeling this option is going to be frequently used for public betas with large pools of testers.