Open Google Calendar, and click the + symbol next to Other Calendars at the bottom of the navigation pane on the left side of your screen.  Click Subscribe to Calendar, then enter the name of the person or group that you want to have available in your calendar list.  Note - It is up to the owner of specific calendars to invite users/groups to their calendars and grant access based on level of detail.  By default, only Free/Busy is available.  The owner has the prerogative of toggling that to Full Details.