Add A User To A Group

In a terminal with root/super user privileges:

adduser username group

For example I wanted to add myself to the groups lp sys & lpadmin to remove a old HP print driver I had installed.

red@red-desktop:~$ sudo adduser red lp
Adding user `red' to group `lp' ...
Adding user red to group lp
Done.
red@red-desktop:~$ sudo adduser red sys
Adding user `red' to group `sys' ...
Adding user red to group sys
Done.
red@red-desktop:~$ sudo adduser red lpadmin
Adding user `red' to group `lpadmin' ...
Adding user red to group lpadmin
Done.