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.  

Enterprise subscription users can be assigned a role.  Learn more about Roles here.

Enable the Notify user via email option to send them an invite email.  Press CONFIRM to continue.