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.
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 email 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.