Setting up a hybrid organization
Directory synchronization is a key part of the integration between on-premises and hosted Exchange because it gives both sides a common frame of reference for user and group information. The initial setup of the Microsoft dirsync tool is pretty painless, but you can make a number of adjustments during the initial setup or later, and it pays to know what those are and to choose your options carefully.
Enabling directory synchronization
Before you can sync your Active Directory domain information with WAAD, you have to tell the Office 365 back end that you want to enable directory synchronization. You do this from the Office 365 portal. Select the Users and Groups tab; an Active Directory Synchronization: Set Up link ...