How do we set it up that expenses claims from employees are emailed to our head office as well as the submitter?

If you want that an email is sent to your head office, you should set his/her email address as the recipient email of the Email Notification. 

Setting up Email Notifications

To send an email to your users once they submit the form you should use an Autoresponder email. 

Setting up an Autoresponder Email

Explanation of Email Notification and Autoresponder Settings