If a User has been previously invited to join their Employer's Network and has not yet accepted that invitation, you will not be able to add them to a Course Offering using that email.
Use the alternative method of adding them with their phone number, or under another email.
Once the user has accepted the invitation from their Employer, reach out to our support team - we can merge the accounts under the primary email address, giving the user access to all of their certificates and training history.