The Managing Users option can be accessed using the dropdown arrow in the Org section (top status bar).
Selecting this option will bring up a screen containing a list of users. You may add or remove users using the controls in this screen.
To Add a User, press the Add User button. This will bring up the Add User dialogue window.
From here you can select any Salesforce user in your organization using the drop-down control. Enable the Notify user via email option to send them an invite email. Press confirm to continue.