It's working again for a couple of our apps but not all of them. I have figured out one workaround: Create a new External Tester group. Add all the people you want to invite. Add the new group to the Available version of the app. It does seem to send out the invites for a new External Tester group.

I was a bit hasty saying it's working again. It is happening with one of my apps today, when adding new external testers to a build which is available and installed by other testers in the same group. The new testers just say "No Builds Available" Does anyone from Apple read these posts? An acknowledgement of the problem would be nice.

It seems to be working again now. I managed to send out a couple of invites. Someone at Apple probably turned it on and off again.