iTunesConnect Testflight no builds available for iOS?
I had this as well. I removed the users from the test group (I did not delete them from TestFlight) and then re-added them. That gave me the "resend invite" option, which I used. The testers received the new invite, and were able to install the build.
I had same issue. For me, I simply had to go under "BUILDS" > click the + mark next to Builds(0) and actually click the check mark next to my build, then it allowed me to "add" the build for my "new" user. I don't know if this is your issue or if it will help, but it was my solution.