I messed up some config files, now all my users have lost sudo priv :(

[Gabriel Dragffy]

Today I did something insanely stupid.

I added a user using useradd (strange that eh?). OK so I wanted to let 
this new user have sudo privs.

What I did was this (don't laugh at me!):
sudo vim /etc/group

Then appended the new user name to the end of the same groups as the 
original user (groups such as adm, and admin). Saved and exited. I try 
to run superuser command with the current user (the original one) it 
tells me I'm not in the sudoers file!?? Change to the newly created 
user, and the same story. Now I've lost all admin privs, and there is no 
way I can fix it until I get to work and reboot in recovery mode.

My question is - how to fix it? What did I actually break?

essentially my group files looked something like:
admin:x:108:originaluser:newuser:

Thanks so much for the help. My life is on the line with this one 
because the server is an essential web and svn server for the company  - 
  argh!!