How to add a shared mailbox in Office for Mac. This is not generally recommended except for administrative assistants or similar circumstances.
"Before you can become a delegate, another person must grant you delegate access to a Microsoft Exchange account. You may receive an email notifying you that you have been granted delegate access to someone's calendar, contacts, or email. To access the items for which you are a delegate, you must add the person who granted you access to your People I am a delegate for list. When you want to stop being a delegate for someone, you need to remove the person from the list." (Source: Office support article).
In Outlook, go to the Tools menu and select Account.
Click on your account (upper left corner) and then click Advanced (lower right corner).
Go to the Delegates section.
In the section Open these additional mailboxes select the + symbol.
Type in a email address to search for the person\shared mailbox. Once found, select the desired entry and click Add.