This code works! Facebook disabled the native console brownser, but if you install the "Firebug" extension and run this script, it will work for you! This script does not delete the group, just remove all the other members and then terminates execution.

facebook-delete-all-group-members.js

// first go to https://www.facebook.com/groups/XXXX/members/
// then paste this in the javascript console
// IF YOU WANT NOT TO INSTALL THE FIREBUG EXTENSION, YOU CAN ACTIVATE THE USE OF THE CONSOLE HERE: https://www.facebook.com/selfxss
deleteAll = [];
deleteAll.elms = [];
 
deleteAll.canClick = function (el) {
  return (typeof el != 'undefined') && (typeof el.click != 'undefined');
}
 
deleteAll.load = function() {
  deleteAll.elms = document.getElementsByClassName("uiPopoverButton _p uiButton uiButtonSuppressed uiButtonNoText");
  if (i < deleteAll.elms.length - 5 && !more[0].hasChildNodes()) {
    setTimeout(deleteAll.getMore, 1000);
  } else {
    setTimeout(deleteAll.go, 2000);
  }
}
 
deleteAll.getMore = function() {
  more = document.getElementsByClassName("pam uiBoxLightblue uiMorePagerPrimary");
  if (typeof more != 'undefined' && deleteAll.canClick(more[0])) { 
    more[0].click();
    setTimeout(deleteAll.load, 2000);
  } else {
    setTimeout(deleteAll.load, 100);
  }
}
 
deleteAll.go = function() {
  try {
    deleteAll.doClick(1);
  } catch(e) {
    setTimeout(deleteAll.load, 1000);
  } finally {
    setTimeout(deleteAll.load, 500000);
  }
}
 
deleteAll.doClick = function(i) {
  deleteAll.elms[i].click();
  setTimeout(function(){deleteAll.doClick2(i)}, 1000);
}

deleteAll.doClick2 = function(i) {
  document.getElementsByClassName("_54nc")[1].click()
  setTimeout(function(){deleteAll.confirm(i)}, 1000);
}
 
deleteAll.confirm = function (i) {
  remove_but = document.getElementsByClassName("_42ft _42fu layerConfirm uiOverlayButton selected _42g- _42gy")[0];
  more = document.getElementsByClassName("pam uiBoxLightblue uiMorePagerPrimary");
  
  if ((i < deleteAll.elms.length - 5 || !more[0].hasChildNodes()) && deleteAll.canClick(remove_but) && remove_but.value == "1") {
    remove_but.click();
    setTimeout(function(){deleteAll.confirm2(i)}, 1000);
  } else {
    setTimeout(deleteAll.getMore, 2000);
  }
}
 
deleteAll.confirm2 = function(i) {
	node = document.getElementsByClassName('uiContextualLayerPositioner uiLayer hidden_elem')[0];
	node.parentNode.removeChild(node);
    setTimeout(function(){deleteAll.doClick(i)}, 1000);
} 
 
deleteAll.load();