1. Login to NOM as the built-in admin user (only one with access to do this).
2. "Settings" tab
3. "Access Control" sub-tab
4. "Add User" link on the left side of the screen.
If you can get away with it, authenticate the users through a domain. That way you don't have to worry about doing password resets, or answering "what's my password" emails/calls. =)