#compdef newgrp

local expl

_wanted groups expl group || return 1

if (( ! $+groups )); then
  if (( ${+commands[ypcat]} )); then
    : ${(A)groups:=${${(s: :)$(ypcat group.byname)}%%:*}} # If you use YP
  else
    : ${(A)groups:=${${(s: :)$(</etc/group)}%%:*}}
  fi
fi

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