How to theme a field group (with div output and css class set)?
I know you mentioned wanting to use the field_group module, but I don't immediately see the need.

If the goal is to position a set of fields all together, why not override your node.tpl.php file, calling it node--my-content-type.tpl.php and then somewhere within it do something along the lines of:

&lt;div id="field-group"&gt;
  &lt;?php print render($content['my_field1']); ?&gt;
  &lt;?php print render($content['my_field2']); ?&gt;
  &lt;?php print render($content['my_field2']); ?&gt;
&lt;/div&gt;


That way they can A) be positioned by positioning #field-group, as well as allowing you to theme each field in their own field.tpl.php file.