Some server hosting providers, such as Linode, start you out with a root account when choosing to use Ubuntu. I think the standard Ubuntu server installer does not create a root account, but instead does the following for you. Anyway, it is noted here that in order to add a user, on Ubuntu, to the sudoers list, all you have to do is the following:
adduser <username> sudoYou do not have to directly edit /etc/sudoers. The /etc/sudoers file is already configured to allow anybody in the sudo group to use the sudo command. Just be sure to make the above adjustment while logged in as root and everything should go smoothly.
— Ryan Cole