Cannot add user "email address already in use"

I cannot see to get this user added into the company. Any suggestions? 

When I login as the company administrator and try to add user, it tells me "email address already in use". 

I went into his office, and had him go to login.teamviewer.com/cmd/joincompany. He did that. He entered the company administrator email address. It told him he was going to lose control of his account, he said sure.

His email still does not show up in the "User Management" tab, and adding him as a user still comes up "email address already in use". The company administartor email account did not get any messages about it.

What can I try next?

Complaining: This is way too hard. As company admin, I should be able to add him, he gets an email and accepts.