Within Teamwork, you can move an existing user to another company.
Note: Only site owners or site administrators can edit a user's company. A site owner cannot be moved and must remain in the owner company.
Navigate to the People section of your Teamwork site and select the relevant user from the list of people.
In the user's profile area, click the options button (three dots) in the upper right corner and select Change Company from the dropdown menu.
You will then have the option to select a new company to move the user to.
Note: Client users cannot be moved to the owner company.
For more information, see: Adding or Editing a Company