Does each user have to create their own firm?

No, new users can be added in the settings/ Users section. 

To add a new user click the grey plus sign in the upper right corner of the Users screen

Enter the desired user's email address.  The new user will receive instructions in their email to complete registration. 

Once completed, the First Name and Last Name fields will update with the new users information.  First Name and Last Name fields will remain blank until the user has completed registration.