I had this issue for my Ubuntu and Pidora, so in this article you can have both solutions:
The problem I had was to add a user on an Ubuntu 14.04 at my work, so that someone else from the company can help me do some basic maintenance of the system (it was requested by my employee!), so this is how I did it after creating a user (new_user):
gpasswd -a new_user sudo
This command will add the user new_user to the group sudo.
I was working with Pidora (which is a Fedora remix for Raspberry Pi) and I was about to run a command using sudo that I faced the following error:
[NixSOS@pidora ~]$ sudo iptables -L -n
[sudo] password for NixSOS:
NixSOS is not in the sudoers file. This incident will be reported.
and I was confused for a while and then I searched for a way to add my user in the “sudoers file” and I’m going to explain how to add a user or a group to sudoers file. This should work all major linux distributions.