How do I add a collaborator who's already a Constant Contact user?

I want to add an existing user of Constant Contact to be a Collaborator to help develop newsletters.  I can't add their email address as I receive an error that the user already exists and to select another email address.  This seems very prohibitive. 

How can I add a person as a Collaborator via the Manage then Add Users that already has a Constant Contact account?