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;
}