Question

I am refactoring some code is a Drupal module I wrote sometime age. In order for others to use it, I am adding a configuration page.

I have successfully defined a fieldset but I don’t know how to ‘insert’ content in to it.
The following code sets up radios for each node type defined on my site:

        $node_types =   node_get_types('names');
    $test   =   array(
        '#title'            =>  t('tweeting node'),
        '#type'             =>  'radios',
        '#options'          =>  $node_types,
        '#default_value'    =>  'Page',
        '#weight'           =>  0,
    );

And the following defines my fieldset into which I want to insert the radio buttons generated above:

        $form['twitterhelper_nodecollection']   =   array(
        '#type'                             =>  'fieldset',
        '#title'                            =>  t('select a node'),
        '#weight'                           =>  0,
        '#collapsible'                      =>  TRUE,
        '#collapsed'                        => FALSE,
        '#parents'  =>  $test,
    );

Answer 1

to add any form element inside the fieldset you should insert this form element inside the field set array …

E.g

$form['myfieldset'] = array( 
'#type' => 'fieldset' , 
'#collapsible' => TRUE ,
'#title' => t('My FIeldset'),
'#attributes' => array('id' => 'myfieldset-id'),
);

$form['myfieldset']['myradios'] = array(
'#type' => 'radios' , 
'#attributes' => array('id' =>'myradio-attributes') , 
....etc
);

so the fieldset is the parent of the radios not the contrast

hop that help you

UPDATE:
you can append the radios inside the field set by using the jquery as the following

jQuery(document).ready(start) ; 
function start(){
  jQuery("#myradio-attributes").appendTo("#myfieldset-id"); 
  // i added this id by '#attributes'
}

but its not the drupal way