A few months ago I submitted a tutorial to Digital Ocean covering FreeIPA server configuration. After a bit of editing, it has been published.
FreeIPA is an open source centralized authentication system made by Red Hat. It’s essentially Active Directory for Linux. It provides a web UI, an LDAP server, certificate management, and an extremely powerful permissions system. FreeIPA even provides a client that allows Ubuntu and Red Hat based distros to easily authenticate against it.
So, if you do need to manage users across multiple linux servers and services, I would really recommend checking out FreeIPA.