I was writing a module to conditionally hide a fieldset on a CCK node editing form based on whether the user is logged in or not, and for a while I was very puzzled as to why my custom module's implementation of hook_form_alter wasn't seeing any of the fieldsets defined by the CCK fieldgroup module.

I searched Google for the terms 'cck fieldset hook_form_alter', which led me to a helpful tip from Benjamin Melançon:

http://agaricdesign.com/note/not-seeing-cck-fieldsets-when-you-form-alter-your-form

The problem is that the CCK fieldgroup module is stored in the Drupal 5 'system' table with a weight of 9. My custom module had a weight of 0. When Drupal runs, it loads modules in order of their weight, so the fieldgroup module wasn't making its changes to the node form until after my module's form_alter() function.

I created an update to bump my custom module's weight to 10 (by implementing hook_update_N()), and after running update.php I immediately gained access to the CCK fieldgroup I had been trying to hide in the first place:

/**
* Fix module weight to be heavier than CCK field group module,
* so hook_form_alter() has access to field groups
*/
function my_module_update_1() {
$items = array();
$items[] = update_sql("UPDATE {system} SET weight = 10 WHERE `name` = 'my_module';");
drupal_set_message('Updated custom module weight.');
return $items;
}

(See "How to update a module's weight" on Drupal.org for more information & discussion.)

In Drupal 6, the Utility module provides an interface for adjusting module weights. There is a Module Weight module for Drupal 5, but at the time of this post it has no stable releases.

As for the original subject of this post, modifying a CCK fieldgroup with hook form alter, here's what I did to hide a fieldgroup from non-anonymous users (an admittedly curious use case, but this logic could easily be inverted to hide a fieldgroup from anonymous users instead):

/**
* Implementation of hook_form_alter().
*/
function my_module_form_alter($form_id, &$form) {
switch ($form_id) {
case 'my_content_type_node_form':
if ($GLOBALS['user']->uid) {
    $form['group_my_group_name']['#access'] = FALSE;
}
break;
}
}