This document shows how allow others within your Exchange organization to access your mailbox folders, including your calendar. This particular documentation shows how to do this using Outlook 2010.
Under the File tab click Info if not already highlighted. Then click and hold Account Settings until the menu appears and click Delegate Access.
In the Search textbox type the name of the person you wish to give permissions to.
Select the name of the person from the list and click Add ->, then OK.
Choose the permissions you would like to give to the person you are allowing access to. For example, if you would like a person to see your e-mail messages, and send messages on your behalf, change the Inbox drop-down menu to Editor (Can Read, Create, Modify Items). After you have chosen the permission settings click OK.