While we usually don't recommend granting external users full access to your Greenhouse Recruiting organization with a full user account, we understand that sometimes external recruiters are hired to consult on specific jobs or your overall recruiting system.
If you've already used a recruiter's primary email to register them as an agency recruiter on the Agencies page, you'll get an error if you try to use the same email to create a new user in Greenhouse Recruiting. Reach out to the agency recruiter for an alternative email address to create a full user account for them.