How do I add a user to the "sudo" group?

sudo usermod -aG sudo <username>

The a is very important. Without it they'll be removed from all other groups. You will need to either restart your shell/terminal or log out and back in for this to take effect.

See also:

• How can I add a new user as sudoer using the command line?

You can either use the user management GUI for it (same place where you create users), or use sudo adduser <username> sudo in the command line.

You can also use a graphical interface. Click on the gear on the top right of the panel, then select "System Settings" and then "User Accounts"

You need to click the little unlock button to be able to edit things in this window. Then click on the person's account and select the proper dropdown for "Account Type"

Its really simple if you are using Unity as your desktop environment.

If you have created a user already then you can simply change it from Standard to Administrator, else make sure that you selected Administrator when creating a new one.

_{Don't forget to unlock before trying to change it}

sudo gpasswd -a $USER sudo

I am late to the party, but this answer might help someone that uses Ubuntu inside a Docker container.

I recently created a Docker container based on Ubuntu 16.04.1.

By default, the Docker Ubuntu image is a stripped down version of Ubuntu, which does not have a vast majority of common tools including sudo.

Besides, by default, the user is logged in to the Docker container as root.

Therefore, I started the container with the docker run command, and installed the 'sudo' package:

root@default:/# apt-get install sudo

Running the command adduser myuser sudo reported error adduser: The user 'myuser' does not exist.. After reading this answer, I first ran the command to create the user:

root@default:/# adduser myuser

Then ran the following command:

root@default:/# adduser myuser sudo
Adding user `myuser' to group `sudo' ...
Adding user myuser to group sudo
Done.

The user myuser was successfully added to the sudo group.

Here's how I setup a non-root user with the base image of ubuntu:18.04:

RUN \
    groupadd -g 999 foo && useradd -u 999 -g foo -G sudo -m -s /bin/bash foo && \
    sed -i /etc/sudoers -re 's/^%sudo.*/%sudo ALL=(ALL:ALL) NOPASSWD: ALL/g' && \
    sed -i /etc/sudoers -re 's/^root.*/root ALL=(ALL:ALL) NOPASSWD: ALL/g' && \
    sed -i /etc/sudoers -re 's/^#includedir.*/## **Removed the include directive** ##"/g' && \
    echo "foo ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers && \
    echo "Customized the sudoers file for passwordless access to the foo user!" && \
    echo "foo user:";  su - foo -c id

What happens with the above code:

• The user and group foo is created.
• The user foo is added to the both the foo and sudo group.
• The uid and gid is set to the value of 999.
• The home directory is set to /home/foo.
• The shell is set to /bin/bash.
• The sed command does inline updates to the /etc/sudoers file to allow foo and root users passwordless access to the sudo group.
• The sed command disables the #includedir directive that would allow any files in subdirectories to override these inline updates.

Use usermod. Add the sudo permission with the following command:

usermod -aG sudo <your username>

Please note that you'll have to use the root account to do this or use another account that has sudo permissions. If you don't have access to another account for some reason and you don't know the root password, you'll need an Ubuntu (or another Linux distro) live CD and then you'll need to chroot into your Ubuntu filesystem and run the above command from inside the chroot.