If TeamViewer tells you, that the email address you wanted to add is already in use, there is an existing TeamViewer account with this address. Do you have access to this email address? If yes, you are able to reset the password via "I forgot my password" and set a new password for the account. With the following link, you are able to join the company with the mentioned TeamViewer account: https://login.teamviewer.com/cmd/joincompany For the joining it's neccessary to know the email address of the company administrator. He will receive a message that the new member wants to join and has to just confirm that.