This article will show you how to add a shared mailbox to your online Outlook inbox. Instructions will be different if wishing to add mailbox to desktop Outlook client.
1. Navigate to https://outlook.office365.com and sign in with account credentials.
2. Right click "Folders"
3. Select "Add Shared Folder"
4. The window below will pop up. Enter the email address of mailbox to add.
The new mailbox will now show underneath your inbox as pictured below.