When adding an existing user to a group, be careful of using ‘usermod.’ If done incorrectly, you can remove a user from its existing groups. In my case, I used ‘usermod’ to add myself to the www-data group. Since I did it wrong, I lost ‘sudo’ access on the next reboot. I ended up booting up from a rescue CD and restoring /etc/groups. Thankfully, Ubuntu keeps a backup copy called /etc/groups-.
sudo usermod -G www-data username
sudo usermod -a -G www-data username