How to modify the registration form?
You can always alter any form by hook_form_alter,in your case it would be somewhat closer to following

// Implements hook_form_form_id_alter().
function my_module_name_form_user_register_form_alter(&amp;$form,&amp;$form_state) {
  $form['help_text'] = array(
    '#markup' =&gt; 'Your help text will go here',
    '#weight' =&gt; $form['mail']['#weight'] + 1; // Or could be a fix value.
  );
}