Adding CSS to extra tpl file
You might be able to do it with css but I might use jquery instead:

{literal}
&lt;script type="text/javascript"&gt;
CRM.$(function($) {
  $('#s2id_groups').hide();
});
&lt;/script&gt;
{/literal}


The problem with css might also be that you don't have a &lt;style type="text/css"&gt; tag around your css?