function MY_MODULE_form_user_register_alter(&$form, &$form_state) {
	$profileFieldset= 'About You'; // CHANGE TO YOUR GROUP
	// cluster all fields into one fieldset
	foreach($form[$profileFieldset] as $key => $item) {
		// push field to account fieldset
		if( substr($key,0,8)=='profile_' ) $form['account'][$key] = $item;
	}
	unset($form[$profileFieldset]);
}

Group all user registration fields into one fieldset

drupal, user

PHP