I have around a 40+ user network running Ubuntu 12.04 Desktop and Ubuntu Server. What is the best tool to use to run authentication on users? Webmin? Similar to what AD does for Windows.
You will probably want to create an OpenLDAP server for clients to authenticate against. Webmin is just a frontend (or rather, it has a plugin that is a frontend) to LDAP. I didn't really find a good GUI frontend for OpenLDAP (if that is what you are looking for).