This will show you how to add another user's inbox via the Outlook Web App.
NOTE: If you are using the Outlook Desktop Application click here.
- Sign into your Outlook mailbox online via www.Outlook.com
- Select your account icon from the top right, and click Open another mailbox

- Fill in the other user's email address and click Open

- This will open up a new tab in your browser with the other users email account open.