Administrators have access to all settings discussed here as well as the management of members and groups of the organization. The easiest way to add Administrators is to share a special invite code.
From the My School page, select Groups at left. In the list of Groups find the group named "Organization name Administrators" Ex: Oak School Administrators. The invitation code will show in Yellow. New administrators can use that code either at registration or enter it on their Groups page.
Note that you'll need to approve them via the 'Management Requests' tab once they've used the code.