Adding fields to user registration form for admins
You could use hook_form_alter to accomplish this

yourmodule_regform_alter($form, $form_state) {
    global $user;
    if($user-&gt;uid!=&lt;whatever&gt;) {
        unset($form['field_attended'];
    }
}


If you want to use permissions, you could create your own with hook_permission

function administer_attendee_permission() {
    return array('set attendee status' =&gt; array(
      'title' =&gt; t('Set status for event attendees'),
      'description' =&gt; t('Set the status for event attendees'),
     );
}


Then your form alter changes to 

yourmodule_regform_alter($form, $form_state) {
    if(!user_access('set attendee status') {
        unset($form['field_attended'];
    }
}