This article was written in the 2013, there may have been some developments in the subject matter.
If in doubt please leave a comment in the bottom of the article.
It can happen for any reason a user (System Administrator) should put his hand to groups in the pc. It can happen for any reason, or program that requires them to change (VirtualBox, Wireshark, etc..). With the new release of Ubuntu, it was decided that the software had reached a maturity that can be administered without user intervention groups, it was decided that the average user does not need to know that there are groups, nor to change them. Remain valid all the terminal commands, but a GUI does not exist.
While I can understand this choice (it is good that the average user does not touch the groups for the mess), other hand, however, I felt mutilated. It is true that I can do everything I want from terminal, but quite frankly I do not have never studied them well related commands on the terminal groups. Give me dell'approssimativo.
The solution is simple.
sudo apt-get install gnome-system-tools
The program is to be started "
users-admin". You can find it under "System Tools" - "Users and Groups".