Your code is fine, minus a capital letter.
Code
<script type="text/javascript">
/* <![CDATA[ */
function addGuest(){
var guestInfo=document.newGuest.guestName.value + ", ";
guestInfo+=document.newGuest.relationship.value;
document.newGuest.guests.value=guestInfo;
}
/* ]]> */
</script>
You had guestInfo += document... as guestinfo +=.
A quick tip, if you're using Chrome go to View -> Developer -> Javascript Console. It will spit back any errors you're getting and help you pinpoint / debug it better.
This post was edited by Blind[zF] on Sep 12 2014 02:58pm