Office 365, Add Shared Mailboxes in Outlook 2016
Follow these steps to open a group account you belong to as a shared mailbox in Outlook 2016. MacOS instructions are at the bottom.
NOTE: By default, group account emails sent or replied to using Outlook 2016 will appear as though they are sent from your email address, not the group account's email address. To remedy this, you must change the From field for the message to the group account's address.
Additionally, know that any group account emails you send using Outlook 2016 will show up in your personal Sent Items folder, not the group account's Sent Items folder. There is no remedy for this other than using OotW to open your group account as a shared mailbox.
1. Right-click on the profile name and click "Account Settings"
2. Click Advanced and then click on the "Delegates" tab
3. At the bottom of that tab, click on the "+" button to add the new mailbox
4. Click OK and then close the Accounts window.
Alternatively, you can also go to File > Open > Shared Mailbox, to open a shared or delegated mailbox.