how to add user to sudoers

How do I allow a user to use sudo?
This is what I have tried:
Code:cia@TRRNTS:~$ su
Password:
root@TRRNTS:/home/cia# /usr/sbin/visudo
[...]
# Allow members of group sudo to execute any command
sudo ALL=(ALL:ALL) ALL
[...]
root@TRRNTS:/home/cia# /usr/sbin/usermod -a -G sudo cia
root@TRRNTS:/home/cia# id cia
uid=1000(cia) gid=1000(cia) groups=1000(cia),24(cdrom),25(floppy),27(sudo),29(audio),30(dip),44(video),46(plugdev),109(netdev),112(bluetooth),115(scanner)
root@TRRNTS:/home/cia# exit
exit
cia@TRRNTS:~$ id
uid=1000(cia) gid=1000(cia) groups=1000(cia),24(cdrom),25(floppy),27(sudo),29(audio),30(dip),44(video),46(plugdev),109(netdev),112(bluetooth),115(scanner)
cia@TRRNTS:~$ sudo ls
[sudo] password for cia:
cia is not in the sudoers file. This incident will be reported.
cia@TRRNTS:~$