1. From your Calendar folder, go to the Home tab > Manage Calendars group, and click Add Calendar > Open Shared Calendar.
    Opening a shared calendar in Outlook    
  2. In the small dialog window that opens, click Name…
    Click Name.    
  3. In the displayed list, find the user whose calendar you wish to add, select their name, and click OK.
    Select the user whose calendar you wish to add.    
  4. If you've selected a valid person, his or her name will appear in the Name box, and you click OK.
    The calendar owner name appears in the Name box.    

That's it! The calendar of your colleague is added to your Outlook under Shared Calendars:
The calendar of your colleague is added to your Outlook.