The "old" GNOME Users and Groups Management Tool has been removed since Ubuntu 11.10 Oneiric Ocelot. The such tool allows us to manage users, groups and their privileges in easy way. I don't know why Canonical removes it from Ubuntu.

Advanced groups management:

Advanced user privileges management:

So, if you want to get this tool back on Ubuntu 12.04 LTS Precise Pangolin, simply run the following command to install:
  • sudo apt-get install gnome-system-tools
Once installed, you can run it from the Unity Dash by searching "users and groups" in the search bar.

Done! Now you have the "old" and advances users and groups management on your Ubuntu Precise box. Enjoy :)