Outlook allows you to add a calendar that has been shared with you, even if you aren't a delegate. If the calendar is not already shared with you, Outlook will display a prompt to send a request to the owner to share the calendar.

Microsoft provides documentation on adding a calendar that has been shared with you.

If you encounter the error message "We can't add the calendar for [name] right now. Please try again later."

1. Navigate to File > Account Settings > Account Settings... > Change... > More Settings > Advanced.
2. Uncheck the box next to "Turn on shared calendar improvements."
3. Click OK, then close the remaining dialog boxes.
4. Restart Outlook.
5. Try adding the calendar again.