I subscribed to this thread and got an email notification for this, just seems that TestFlight has took a holiday. Come on Apple, we all need you to fix this!

I'm having the same issue as well. I've tried both personal and business emails and cannot for the life of me get a testflight invitation for even myself. I have other apps on testflight that I can get new build updates just fine. It's just establishing new testers seems to be an issue