sudo problem

[Bob]

I am a new Ubuntu user.  I installed virtualbox and was trying to configure it.
I added my userid (using usermod) to the virtualusers group so I could access
USB devices from virtualbox, USB devices now work.

Sudo worked to add my userid to the virtualbox group but did not work
afterwards.  I don't know if adding my userid to the group caused the problem
or not but now I get an error message when I use the sudo command.

[sudo] password for bob: 
bob is not in the sudoers file.  This incident will be reported.
bob at Saturn:~$ 

I don't see anything in the sudoers file that would have anything to do with
this problem.

The /etc/group file has 'sudo:x:27:'.  Can I edit that file (using the Ubuntu
CD) and add my userid to make it 'sudo:x:27:bob' to fix this problem?

If not how do I fix this problem?

-- 
Robert Blair