running sudo command for a user in centos

On a Centos-6.1, the user is in the root group, but he is not able to run sudo commands

mahmood@cluster:~$ groups
root kvm vboxusers google-otp mahmood
mahmood@cluster:~$ sudo ls
[sudo] password for mahmood:
Sorry, user mahmood is not allowed to execute '/bin/ls' as root on cluster.scu

Any idea on that?

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