At the end of the Installation Docs in the Finalizing Your Gentoo section, it gives an example useradd command that will fail if entered, as the -n does not work and prints the --help info. Reproducible: Always
Please give a URL. The nearest page I can guess you're referring to is: http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?part=1&chap=11 This uses -m, not -n. -m is necessary as it tells useradd to create the home directory and populate it with the default content (from /etc/skel)
Allen's correct -- you just typed it wrong. Not a bug; closing.