Creating group, giving sudo privileges and adding user to the group

Step 1: Login to root account
..$ su

Step 2: Adding user
..# useradd -m frank

Step 3: Creating group
..# groupadd rangers

Step 4: Edit sudoers and add rangers into
..# vim /etc/sudoers

# Allow members of group sudo to execute any command
%sudo   ALL=(ALL:ALL) ALL
%rangers  ALL=(ALL:ALL) ALL

save and exit

Step 5: Adding frank to rangers

..# usermod -a -G rangers frank

Finished.

Now you have a user frank in the rangers group, which was added to sudoers file.

You should able to use sudo while you are frank.