If you need to shut down you server and you need to notify your users of this you can send a message to there screen notifying them of a server reboot or shutdown. This article covers windows and unix /linux.
A step by step guide on how to add a user to be a Exchange 2013 administrator. How to add users to Exchange 2013 managment roles. A full list of Exchange 2013 roles.This guide also applies to adding an administrator to Office 365 Exchange