If you need to appoint someone to multiple entities at once, navigate to the People tab and then click on the person's record that you want to resign. The click on the Interests section where you will find a list of all that person's Principal appoints.
Click on the New button to add that person to the desired offices in the desired entities.
This appoint workflow is identical to the regular appoint principal workflow. Please follow this help article for more detailed instructions on how to navigate the appointment wizard: https://help.athennian.com/en/articles/3156547-appoint-a-director-officer-or-manager