New users can only be added by the admin of the account.
To do so, you must go to "Admin" (upper right corner):
Under "User Management", an additional user can be added:
Simply enter the name and email of the new user and submit the request. An email with an invitation and corresponding registration link will be sent automatically to the new user.
By registering with the email address specified by the admin, the new user will be linked automatically to the business account.