To be able to shutdown or reboot your system, you'll have to add .rules
files in %%LOCALBASE%%/etc/polkit-1/rules.d directory. Which looks
like this (replace PUTYOURGROUPHERE by your group):

polkit.addRule(function (action, subject) {
  if (action.id == "org.freedesktop.consolekit.system.restart" ||
      action.id == "org.freedesktop.consolekit.system.stop"
      && subject.isInGroup("PUTYOURGROUPHERE")) {
    return polkit.Result.YES;
  }
});

For those who have working suspend/resume:

polkit.addRule(function (action, subject) {
  if (action.id == "org.freedesktop.consolekit.system.suspend"
      && subject.isInGroup("PUTYOURGROUPHERE")) {
    return polkit.Result.YES;
  }
});