Once you know how to do it, adding an existing user to an existing group from the console is a lot faster than using any GUI. To add the user
exit to the group
adduser exit mail
To remove the user
exit from the group
deluser exit mail
Both these commands require administrative privileges (obviously). So call them with
sudo or from a
root shell. If you’re changing your own groups, you will need to log out and in again to see the changes.
Note that there is a related command,
usermod -G mail exit, but it probably won’t do what you want, since it removes the user from any groups he is part of, and adds him to the group
usermod -G mail,admin,floppy,cdrom,scanner,audio,video exit
but it’s a lot clumsier if you ask me.