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 how to manage 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 in the main menu:

The name of the company is displayed under your profile name so you could quickly see which Bïrch account you're currently in. To switch between the companies just select the company you're invited to from the list and click on its name.