How to modify forms and set defaults

In many cases like with Organic Groups, you want to remove some fields, make tham invisible and set defaults.

The best approach is to create a site-specific module and write a hook_form_alter function.  Do a print_r($form) to find out the form structure and then unset the fields that you want to zap or change the fields properties.

To check a checkbox use $form['og_private']['#attributes'] = array('checked' => '1');

Example:

function yoursite_form_alter(&$form, $form_state, $form_id) {
    global $user;
//    echo "form_id = $form_id <br/>";
//    print_r($form);
//    foreach ($form as $key => $value){
//        echo "$key - $value <br/>";
//    }

    // Set the defaults for the group creation.
    if ($form_id == 'customer_node_form') {
        unset($form['og_selective']['#options'][1]);     // hide Moderated option
        unset($form['og_selective']['#options'][2]);     // hide Invote only option
        unset($form['og_directory']);        // hide List in groups directory
        unset($form['og_register']);         // hide Registration form
       
        // Check "Private group" and disable it.
        $form['og_private']['#attributes'] = array('checked' => '1');
        $form['og_private']['#disabled'] = 1;
    }
}

One issue that you might encounter is that if you want to assign specific properties to radio button you'll find that in the hook_form_alter function they haven't been expanded yet.  The solution is to register a funtion that is called after the expansion.

function yourtheme_form_alter(&$form, $form_state, $form_id) {
   $form['og_selective']['#after_build'][] = 'yourtheme_select_closed_group';
   ...
}
function yourtheme_select_closed_group($element) {
  // the radio has been expanded by now
  $element[3]['#attributes'] = array('checked' => '1');
  return $element;
}