You are calling the colorBox()
function twice which I think is incorrect.
If you combine those you should be good to go:
<a id="openGallery" href="#">Click for more images</a>
<script type="text/javascript">
$(document).ready(function() {
//just one call to set up the colorbox
var $gallery = $('a.slideGroup').colorbox({rel:'slideGroup', transition:"elastic", opacity:"0.5"});
$("a#openGallery").click(function(e){
e.preventDefault();
$gallery.eq(0).click();
});
});
Here is a working example on CodePen: http://codepen.io/simoncowie/pen/agxGl