Office 365 (Outlook 2016) - Open a Shared Mailbox to Perform Mail Merge
Follow these steps to open a group account to which you belong as a shared mailbox in Outlook 2013. This method does not add the account to your existing Outlook profile. This will allow you to view only the shared mailbox or only your regular account mailbox. This method is required if the user intends to send Mail Merge emails from the shared mailbox.
Open the control panel (click on the start button and type control, click on control panel)
Type mail in the search bar and click on mail (32bit)
Click on Show Profiles...
Make sure that Prompt for a profile to be used is selected, and click add
Give the new profile a name, preferably the name of the shared mailbox
When prompted for an email address, your email address will be shown. Delete this address and substitute that of the shared mailbox remembering to give it a Name as well. Leave the password blank for now and click next.
You will be prompted for a password. Use your email address and you may choose to check Remember my credentials
Click finish when it confirms completion, and then click ok to exit the mail configuration window
Launch Outlook and choose your newly created profile for the shared mailbox.
The shared mailbox will now open in Outlook using the rights of the username entered above.