Users can be invited to several companies with the same email as a regular user or as an admin on the Settings page:
Check out this article on managing team member permissions.
Once the user is invited to a new company, they receive an email with the invitation link to join the company.
The list of all companies you’re invited to can be found by clicking on the workspace icon in the top left corner:
To switch between the companies, just select the company you're invited to from the list and click on its name.