Use the usermod or adduser command to add user to sudoers group
sudo usermod -aG sudo username
or
sudo adduser username sudo
To remove the need to enter password when issuing sudo (privilege) commands. The file that the configure is saved /etc/sudoers. Use the visudo command to edit the file
sudo visudo
Make change to the following line.
%sudo ALL=(ALL:ALL)
# Change it to the following to disable the need to enter password for all sudoers users.
%sudo ALL=(ALL:ALL) NOPASSWD:ALL
# You can also specified a specific user to remove the need for password.
demo ALL=(ALL:ALL) NOPASSWD:ALL