The old Gnome User and Groups management tool is not available in Ubuntu 12.04 LTS.
There is User Accounts available under the System setting, but you cannot administer groups.
You can solve this problem by installing the utility gnome-system-tools which is available in Ubuntu software repository.
How to install gnome-system-tools
Open a terminal and run the following command . or install it using synaptic Package manger.
$ sudo apt-get install gnome-system-tools