If you'd like to add a user onto your diary, you can do this through your company settings. Scroll down to 'Users', this will bring up a list of your current users. Click 'View' next to the user that you wish to add. Scroll down to the 'Business details' section and click 'Edit business details'.
From here, you can select whether or not you'd like your user to appear on the diary, you can change this setting at any time.