You may encounter this error when installing packages that manage system groups if your installed version of shadow is older than version 4.1.4.2-r4:

>>> Installing (1 of 27) net-misc/dhcp-3.1.2_p1
 * Adding group 'dhcp' to your system ...
 *  - Groupid: next available
groupadd: invalid option -- 'r'
Usage: groupadd [options] GROUP

Options:
  -f, --force                   force exit with success status if the specified
                                group already exists
  -g, --gid GID                 use GID for the new group
  -h, --help                    display this help message and exit
  -K, --key KEY=VALUE           overrides /etc/login.defs defaults
  -o, --non-unique              allow create group with duplicate
                                (non-unique) GID

To fix, update shadow before continuing with your installation or upgrade:

# emerge --update shadow

Depending on the state of your system this may also update your PAM installation. It might be a good idea to run etc-update before moving on.