How to I add another User's mailbox to my favorites in outlook 2013?

You can add a mailbox to your Navigation using the following method:

Go to your mailbox name in the Navigation pane, right click and choose "properties"

Click on the "Advanced" button

and then the "Advanced" Tab

Click the "Add" button, and type in the name of the account.

Press OK, and then press OK again. Check that the new mailbox has shown up in your Navigation pane. It will usually be below your main mailbox.