A separate dedicated account should be created for your shared calendars. This is necessary for permissions to work properly.  This user must be in contacts as well. By creating the account, the user will be added to your Global contact list and then accessible if you have contacts turned on for your own account. The process is explained below.

Through OWA or Windows Outlook. This must be done on OWA or Windows Outlook.

1. Log into the new OWA for your newly created Shared Calendar account. 
2. Select Calendar button on bottom left.
3. Create calendars for sharing.
4. Right click on the calendar you want to share and select sharing permissions.
5. This will trigger an invite to your calendar.  Enter the email(s) for the people you want to invite. 
6. From the drop down menu below, select the permission level for this calendar.
7. Click Share, sending an email invitation to the user(s).

1. A sharing invitation is received.
2. This invitation needs to be accepted on either macOS or iOS device.

For macOS:

1. Go to System Preference, Internet Accounts. Select your Office 365/Exchange account
2. Turn on Contacts and Calendars (This step is crucial and sharing will not work without it.)
3. Launch Contacts and sync contacts local on your Mac
4. Launch Calendar 
5. Go to Calendar Menu, Preferences
6. Select Accounts
7. Select Delegate
8. Add User Email address for user that sent you the invite to share.
9. Check the box to Show. 
10. Close Calendar Preferences
11. The Shared Calendar appears in Calendars and on your iOS devices.

For iOS

1. Refresh calendars in iOS. by dragging word Calendars down.
2. If you have a cache issue with older calendars, you can delete calendars by Tapping i and selecting Delete at bottom.
3. Go to system preferences and turnoff calendars. 
4. Reenable the calendars to start fresh.