#compdef newgrp

local expl

if (( ! $+groups )); then
  if whence -p ypcat > /dev/null; then
    : ${(A)groups:=${${(s: :)$(ypcat group.byname)}%%:*}} # If you use NIS
  else
    : ${(A)groups:=${${(s: :)$(</etc/group)}%%:*}}
  fi
fi

_description expl group
compadd "$@" "$expl[@]" - $groups