Step One: In Outlook, find Calendar at the bottom left of the client and click on it.
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Step Two: Under the Home tab, locate the Share section and click Calendar Permissions.
Step Three: A new box will come up called "Calendar Properties".
Step Four: Click Add. You may now select the user(s) you wish to have access to your calendar. You may also select their respective permission levels (Reviewer, Author, etc.)
Note: While Outlook will allow you to add users via email address it will not work. You must search for them and select them from within the Address Book as shown below.
Step Five: Click OK when finished.
Opening Shared Calendars
The users who have just been assigned access, will now need to open the calendar in their Outlook client.
Step One: Right click on My Calendars, select Add Calendar, and then From Address Book.
Step Two: When the following window appears, search for and add the person who's calendar you are trying to access.
The calendar will appear in the list of calendars on the left side of the window.