Hello @TBEManatee ,
Any email address can potentially be verified on your account, for use as the main contact email, billing address, etc.
However when adding as a user, the initial username for the new user will be the email address being utilized. The messaging being received is because the person you're trying to add has their email address as a username for another account they're on, it may even be an expired trial they setup awhile ago. 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, or if they're unable to find the account they originally setup under that address, 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.
Community & Social Media Support