In Outlook -> Calendars -> Home tab, choose "Add Calendar +", and click "Open Shared Calendar..."

In the Search box, look up a person's name, NetID, or calendar name and click "OK"

If there are multiple options, select the calendar you want to open and click the "Open" button

The calendar will open and now be listed on the left-hand column and can be toggled on and off with a checkbox

Note: This function is in a different location in the "New Look" for Outlook for Mac. You can find the adding Shared Calendars section under File -> Open -> Shared Calendar

For the "Legacy/Old Look" follow the steps below:

In Outlook -> Calendars -> Home tab, choose "Open Shared Calendar"

In the Search box, look up a person's name, NetID, or calendar name

If there are multiple options, select the calendar you want to open and click the "Open" button

The calendar will open and now be listed on the left-hand column and can be toggled on and off with a checkbox