Username appears in parenthesis, can't log in (Ubuntu Server)

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http://askubuntu.com – We have been setting up a server in my networking class using Ubuntu Server. I SSH'd into it using putty and made an account. Then, I began messing with it, trying to put my name in the sudoers file. I believe I succeeded, but, when I went to the server itself, which now has the desktop GUI installed, it showed my name, and the name of another user who did the same thing as me, in parenthesis. When either of us logged in, it flashes some code real quick then takes me back to the log in screen. I believe we're using 12.04. Thank you. (HowTos)