All of the openssh ebuilds in portage contains the command "enewgroup sshd 22". Which makes use of groupadd which belongs to shadow on my system. Therefore they fail with "groupadd: command not found" if shadow is not installed which was the case on a new stage3 system when I emerged -e system and it attempted to install openssh before shadow.
*** This bug has been marked as a duplicate of 113298 ***