If people share their calendars with you, they will have sent you a link through email. Simply click on the link and the calendar will show up in 'My Calendars' on the left-hand side of the calendars page when you view it.

You can also add calendars using email by going to the OTHER CALENDARS option on the left-hand side and where it says 'Add a friend's calendar' or 'Add a coworker's calendar' simply enter the person's email address and hit enter.  If it's shared, you'll see it. If not, a request will be sent via a form where you will enter the email address. If they don't USE Google calendar, you'll see an option to invite them to do so.

You can also add a PUBLIC calendar. Go to the OTHER CALENDARS on the left-hand calendar menu and click the down arrow - then choose the ADD BY URL option. Enter the calendar's URL. The address needs to be an ICAL link - as defined by the Google help option below.