To open or add a calendar to your Office 365 account, you can use the following procedure.

Login to https://mail.gatech.edu using your GT credentials.  Then, select the grid at the top left and select Calendar.  

Near the top of the screen, click  Add calendar -> From directory.

Search for the user or calendar name and click Open.  This will add the calendar to your account.  In the following picture, the search term used and calendar added was "TSRB222".