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.  

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

Enable the Account admin option to allow user to manage subscription, users, roles, sandboxes and credits.

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

Press CONFIRM to continue.