ZOHO One new user: Email address is already used

ZOHO One new user: Email address is already used

Hello, 

I am trying to add david "at" rockrobotic ".com" as a new user to my Zoho One - "Email address is already used" 

I have checked with the email address and David is not currently a user of zoho.  Can you help remove his email from any other organization so I can add him to our account?