http://askubuntu.com/questions/168280/how-do-i-grant-sudo-privileges-to-an-existing-user
sudo usermod -a -G sudo <username>
-a, --append
Add the user to the supplementary group(s). Use only with the -G option.
-G, --groups GROUP1[,GROUP2,...[,GROUPN]]]
A list of supplementary groups which the user is also a member of. Each group is separated from the next by a comma, with no
intervening whitespace. The groups are subject to the same restrictions as the group given with the -g option.
If the user is currently a member of a group which is not listed, the user will be removed from the group. This behaviour can be
changed via the -a option, which appends the user to the current supplementary group list.