Using Outlook for iOS for personal email and calendar management
I tried the activity monitor workaround, but it doesn't work with the latest macOS. Instead, you'll need to add the calendar as a subscription calendar if it doesn't connect through logging into your Exchange account.
Steps:
1: Go to outlook.com (it doesn't work in the application).
2: Open Outlook.com's Settings (Gear icon in the upper right)
3: Click "View all Outlook settings"
4: Click "Calendar"
5: Click "Shared Calendar"
6: You should see all of your shared calendars. Under "Publish a Calendar", copy the ICS link.
7: Open up the iCal app.
8: Click "File". Then click "New Calendar Subscription..."
9: Paste your ICS link and subscribe.
10: Give it a minute, and your Outlook calendar will populate on your iCal.
You'll have to do this with each of your shared calendars in Outlook. Hope this helps!