By default, users with the permission of 'administer users' cannot assign roles on the user registration form. With this snippet you can, but make sure to hide the 'administrator'-role for your users.
/*
* Allow adding roles in user registration form
*/
function MYMODULE_form_user_register_form_alter(&$form, &$form_state, $form_id) {
unset($form['account']['roles']['#options']['administrator']);
$form['account']['roles']['#access'] = TRUE;
}