The email cannot be added as a Customer because it needs to be added as a User

The email cannot be added as a Customer because it needs to be added as a User

Hi,

 We are experiencing issues with adding another portal user.

 The portal user has the same domain as 22 other users but we cannot seem to add anymore portal users with the same domain.

"The email cannot be added as a Customer because it needs to be added as a User. Please contact support@zohocreator.com for further assistance."