Click Add a colleague's calendar on the left panel and start writing your colleague's name or email address. The system suggests calendars based on what you write. When you find the person you'd like to add, just click the suggestion and the calendar will be added.
If your colleague has opted to share his or her calendar publicly, it will then appear under 'Other Calendars' in the left column. If not, you'll see a message requesting access to your colleague's calendar. Feel free to add a note to the request, then click Send Request.