Adding an already-registered email to an existing account

Hi, We wanted to add someone to our Constant Contact account. The person has a Constant Contact account (which they created by mistake). We sent them an email request to be added to our account, but when they clicked on the "verify email" button in the invitation we sent, they got logged into their own account. They don't see our campaigns. How do we fix this? There doesn't seem to be a way to delete the account unless you call the company. Is that the only way? If that's the case, does it mean that if I have an existing account, I can't use that same account as a member of another account? Thanks, L