You can add new users to your company either by clicking the 'Add new user' link under the administration tab from the user overview page or by clicking the Add user button in the User list page. User list page can be visited by clicking the User list under the User accounts menu.

In the 'Add user' Page fill all the necessary details and then click 'Add user' to add new user. The user added by you will be inactive until the payment for an additional user is done. The email address is necessary for every user.