From: Open a shared Mail, Calendar or People folder in Outlook for Mac - Microsoft Support
Open a Shared or Delegated Mailbox Mac Outlook Desktop App
Use this method if you are a delegate for another user’s mailbox or when you have permissions to several folders in a shared mailbox. You can also use this method when you have full access permissions to a mailbox.
To open the shared mailbox:
In the Tools menu, choose Accounts, and select the account that has access to the mailbox.
Select Delegation and Sharing
Choose Shared With Me tab
Choose + to add a shared or delegated mailbox