This article will show you how to add a shared mailbox to Outlook 365 Desktop or Web. The mailbox must be shared with the user prior to adding it to Outlook.
Outlook desktop client
The steps and images below are using the Outlook for Office 365 application on a Windows PC.
The mailboxes can be added by going to File > Info.
Select Account Settings, then select Account Settings a second time.
A box will pop up showing the account of the user. This account must be selected.
Click Change.
Click More Settings.
Click Advanced.
Click Add.
A box will pop up asking the account to add.
Type in the email address of the account you want to add.
Click Ok.
The account should now be listed in the left-hand folder navigation bar within Outlook.
Outlook web
In Outlook on the web, in the left-hand folder navigation bar, right-click and select Add shared folder.
Type in the email address of the account you want to add.
Click Add.
The account should now be listed in the left-hand folder navigation bar within Outlook Web.