I added a person in iTunesConnect (User and Roles) with the role as developer.
After he accepted the invitation, he is ready to be added to test the build. I could add the user to my particular app build. Then I tried sending an invitation to him. But the status of the user is "-".
Please refer the screen shot.
Earlier I could add number of testers, but why am I getting a problem only with this user?
Try removing the user, re-add him and Resend the invitation. It always works in my case.