apt-get install sudo visudo [username] ALL=(ALL) ALL kennyl ALL=(ALL) ALL

First... make sure the sudo package is installed. If you execute visudo and get -bash: visudo COMMAND NOT FOUND then the package is probably not installed.

Execute this...

SUDO is an abbriviation for Super User DO. Being part of SUDO is like being in the Super Users group. To include a user in this group, login as root and use:

Add the new user right below the line root ALL(ALL) ALL like this:

Where username is the name of the actual user i.e. kennyl.
Example:

I believe the user will be active on the next boot and or login.