Open a shared mailbox in Outlook for Mac

Instructions

From the Tools menu, choose Accounts.
In the window that opens, select your account and click Advanced.
Go to the Delegates tab and under People I am a delegate for, click the + sign.
In the Search field, type the email address you are trying to add. Select it from the list and click Add or OK.
Click OK to get back to the accounts window and close this window.
If you see the login screen enter your email address and password. If you see a message about being redirected to the exchange server, check Always use my response for this server and click Allow.
The shared account will appear in the list of folders below the folders for your personal account.

Please note that all mail sent from this account will be placed in the Sent Items folder for your individual account.