Step 1: To start, go into the File tab > Account Settings. Once you click on Account Settings, click the first option: Account settings.
Step 2: Once done, a dialogue box will appear titled Account Settings. Click the Change button, and then another window will appear on the screen.
Step 4: Click More Settings, which will open up another window titled Microsoft Exchange. Open the Advanced tab > Add, and then type the full account name of the person who you wish. Once done click OK. Then click Apply, to apply all your settings and you're all set. You should see the additional inbox in your Folder Pane located on the left hand side when you're done.