For Outlook on Desktop

1. Open Outlook and go to Calendar view
2. Click Add + (simple ribbon) or Open Calendar + (classic ribbon)
3. Click Open Shared Calendar…
4. Enter the address of the shared calendar and hit enter / click OK

For Outlook on the web (OWA)

1. Open Outlook on the web (OWA) and go to Calendar view
2. Click Add calendar in the lower left side menu (above your list of calendars)
3. Choose Add from directory
4. Select your own primary email account
5. Enter the address of the shared calendar and hit enter click Add