Directions for this process are outlined on this website under the section for linking to a public calendar:

Important: You can only add a calendar using a link if the other person's calendar is public. Learn more about public calendars.

1. On your computer, open Google Calendar.
2. On the left, next to "Other calendars," click Add   From URL.
3. Enter the calendar's address.
4. Click Add calendar. The calendar appears on the left, under "Other calendars."

Tip: It might take up to 12 hours for changes to show in your Google Calendar.