Within Teamwork, you can add users to a specific company when adding them to your site.
The owner company will be selected by default, however you can choose to add a new external company or choose an existing one.
Note: A user can only be part of one company at a time.
To add a new user or update an existing user's details, go to the People area of your site.

Adding a new user to a company
To add a new user, go to the People subsection of the site-level People area.

Click the arrow beside the Invite Users button in the top right and select Add User from the dropdown menu.

You can choose which company to add them to in the Company dropdown in the Essentials tab.

Updating an existing user's company
You can also update an existing user's company via the People subsection of your site. Click the three dots to the right of the relevant user and select Edit Profile from the dropdown menu.

For more information, see: Companies