Hello @RobD102 ,
They'll need to login to the other account they own / are a user on, and update their username to something different / unique. Once they've done that you can add them as a user to your account using that freed-up email address.
It'd also be advisable that once they've gotten their login setup for your account, they update their username to something more unique - that way they're free to have their email address be used to setup another account user login if necessary.
If they need further assistance with updating their other accounts' username, I'd advise having them call general support to provide the email address in question, confirm security permissions live, and work with the agent to administratively update that other username.
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