How can i load a table on changing a select option using AHAH callback
Try to add some wrapper to your table:

function list_user_form(){
   $form['howmany'] = array(
      '#title' =&gt; t('List all the users in a group'),
      '#type' =&gt; 'select',
      '#options' =&gt; select_usergroup_dropdown_options(),
      '#default_value' =&gt; $default,
      '#ahah' =&gt; array(
         'path' =&gt; 'mailusers/listuserstable',
         'effect' =&gt; 'fade',
         'wrapper' =&gt; 'mytable'
      )
   );

   $form['mytable'] = array(
      '#title' =&gt; t("Generated table"),
      '#prefix' =&gt; '&lt;div id="mytable"&gt;',
      '#suffix' =&gt; '&lt;/div&gt;',
      '#type' =&gt; 'fieldset',
      '#description' =&gt; t('This is where we get automatically generated table')
   );

   $form['submit'] = array(
      '#type' =&gt; 'submit',
      '#value' =&gt; t('Click Me')
   );
   return $form;
}

function listuserstable(){
   // make html for the table and rendering it
}