Added one file under /etc/sudoers.d named "pivtl.conf", the content is:
%pivtlusr       ALL=(ALL)       NOPASSWD: ALL

It should give user in pivtlusr group sudo permission to all command without the need of password.
But it DOES NOT work!? Why?!!!!!

Then I found this link: https://bbs.archlinux.org/viewtopic.php?id=141539
A member "fwolf" helped with the following content:

 "Just a note: 
sudo will read each file in /etc/sudoers.d, skipping file names that
end in ~ or contain a . character ...
This solve my sudoers.d/ file not working problem, found here:
http://bugs.centos.org/view.php?id=5017
Hope this helpful.
Then I changed the name "pivtl.conf" to be "pivtl", everything works like a charm!
Anyway, good to know this!