Add this to the top of your headers if you don't have it:
<script src="/jquery.js"></script>
Footers
<script>
$("td[width='16%'] b a").each(function(){
if(this.href.match(/\?action=profile;u=(\d+)/i)) {
//Created by Agent Moose (smcodes.smfforfree3.com)
var number = RegExp.$1;
$(this).parent().next().find("img.avatar").after("<br>Member Number: "+number)
};});
</script>
Basicly what the called is

It will show what number each member was when they joined.
EDIT:
Since that one only worked for showing it under the AVatar, if you had one, I made one that will show the number under there username

<script>
$("td[width='16%'] b a").each(function(){
//Created by Agent Moose (smcodes.smfforfree3.com)
if(this.href.match(/\?action=profile;u=(\d+)/i)) {
var number = RegExp.$1;
$(this).parent().after("<br/><span class='smalltext'>Member Number: " + number + "</span>");
};});
</script>