In Linux, if the user that you created want to use sudo privileges, you should add this user to /etc/sudoers(add something like jack ALL = (ALL) ALL). Well, the default mac user(the one you login to mac when you turn on you Mac) can use sudo too, but it's not in /etc/sudoers file, but root user is in it, do you guys know how mac perform this?

As far as I know, the user I mentioned above is not in /etc/passwd too, I found it in /private/var/db/dslocal/nodes/Default/users/XXX.plist, xxx is your user name.