On App Store Connect, when a new TestFlight build is uploaded, it's no longer possible to add a group to the build. (FB14892147)
I've seen this error message for 2+ hours.
Other developers are reporting this issue: https://x.com/manuel_kehl/status/1826678066392793188
Apple's developer system status page doesn't report an outage: https://developer.apple.com/system-status/
Thank you for investigating this issue.