If you are creating a new participant user, you will be required to enter in the First Name, Last Name and Email Address. The participant's email should be unique.
If you try to create a new participant and receive an error message that the "Email Has Already Been Taken", that means this user is already in the system.
First, check to make sure that the user has not already been added to your organization's Participants list by another lead.
If they are not already in the list, their email could be associated with another organization that utilizes the CCAT Survey. If this is the case, you will need to use a different email for this participant. That participant will need to log in twice, using both emails, in order to take the CCAT Survey for each organization.