installation issue on linux - asking for valid user group?

when I begin the install on my linux server... I get past the point where I enter the admin name and password.. then it ask me to give an alternate user ID (default is nobody) ... after doing so - the install script ask me for a valid user group for the user.. I've tried everything, and cant figure out what a valid user group is!!! and there is no documentation in the install guide that even refers to this question ever being asked... if someone could help me out with this - it would be appreciated - because the installation wont go any further until I answer this question