I am having the same issue, just started to experience the last week. I have not experienced this before. I have confirmed the user is not added to the user list various times but I still cannot add them. It's very frustrating. We were also having issues allowing external testers download a beta version, which is why I was trying to add a developer on a different team to the developer team to allow them to download the latest a bit faster. Any help would be appreciated. This is affecting feature development so quick help on what is happening would be appreciated ❤️