Great question! No information is provided by TestFlight to the developer by joining a test group. When submitting feedback such as crash reports, or screenshots, then only anonymous device information is provided such as iOS version, device type, connection type, time zone, battery level, free disk space, and screen size. Optionally users can also provide their email address when submitting feedback so the developer can contact them, but this is not required.

There are also private TestFlight groups where you need to share your email with the developer so they can specifically add you to the test group, but that is the only difference between private and public groups.

So you can rest easy, you’ve not been sharing any information just by joining a public TestFlight.