When trying to add a user in Control Hub you may see the following error: 'The user already exists in another organization'.
The most common cause for this error is the affected user has a free consumer account for Webex or Webex Meetings.

To resolve the issue, delete the free consumer trial account: Delete a Customer Organization in Cisco Webex Control Hub

 Once the free user account is deleted, try adding the user again in Control Hub.
Note: A user can not exist in multiple org with the same email.