Within Teamwork, you can move an existing user to a new company.
Note: Only site owners or site administrators can edit a user's company. A site owner cannot be moved to another company. They must remain in the owner company.
To make these changes, navigate to the People section of your Teamwork site.
Select the user you wish to move from the list of users. Then to the top right of their details page select the options button (three dots) and then Change Company.
You will then have the option to select a new company to move the user to.
For more information, see: Adding or Editing a Company