This guide demonstrates how to open an existing shared mailbox within Outlook 2016 for Mac.
- From the top menu, select Tools. Select Accounts…
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The Accounts window will be displayed. Select the Advanced… button.
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Click on the Delegates tab. Beneath the people I am a delegate for section, select on the plus (+) button.
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Enter the shared mailbox name or email address within the provided search field and select the correct result. Select Add.
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You may be provided with an auto-discover prompt requesting that you allow the remote server to configure settings within Outlook. Select Allow.
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The shared mailbox account should now be added to the People I am a delegate for list. Select OK.
 - Your shared mailbox should now be added to your folder list on the left-hand side of your Outlook client. You have now successfully added your shared mailbox.
 
Alternative Method
- Open Outlook for Mac.
 - Select File.
 - Select Open.
 - Select Other User's Folder.
 - Select Inbox from the folder type drop-down menu.
 - Enter the Title of your shared folder in the search box.
 - Select your Shared Mailbox from the list.
 - Select Open.