Add commas to a number HTML DOM

I hope, I'm phrasing this right. I'm grabbing a value from a XML file, and I would like to add commas every thousands value (1234 -> 1,234 and 1234567 -> 1,234,567 and so on.)

Here is the script I'm using, borrowed from the w3schools website (I'm VERY new to javascript).

 if (window.XMLHttpRequest)
   {// code for IE7+, Firefox, Chrome, Opera, Safari
   xmlhttp=new XMLHttpRequest();
   }
 else
   {// code for IE6, IE5
   xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
   }
 xmlhttp.open("GET","http://ift.tt/1sah8Dw",false);
 xmlhttp.send();
 xmlDoc=xmlhttp.responseXML; 

 var x = xmlDoc.getElementsByTagName("groupDetails");
 for (i=0;i<x.length;i++)
   { 

   document.write("<b><font size='3' color='#00a0fe' face='Arial'>");
   document.write(x[i].getElementsByTagName("memberCount")[0].childNodes[0].nodeValue);
   document.write("</b></font>");
   }

This is all being done through the script file, and not the html file. I've been through a few similar answers across the site, but was never able to apply the answers to my script. Hopefully someone can at least point me in the right direction.