Under "testers & groups", I only have "all testers" and "iTunes connect users". There is no "+" for adding testers in "all testers" and the "+" in "iTunes connect users" is only for adding internal testers. I don't have any groups shown nor do I have the option to add a group.
I had external testers before (and they still appear in "all testers") but I did not create any groups.
Maybe the problem is that I have not uploaded any builds yet?