function usersfilter_form_alter(&$form, &$form_state, $form_id) {
     global $user;
     if (in_array('administrator', array_values($user->roles))){
        return; //administrator
     }

    $newUsers = array();
    $existingUsers = $form['field_assign_to']['und']['#options'];

    foreach ($existingUsers as &$existingUser) {
        if($existingUser == $user->name){
            $newUsers[] = $existingUser;
            continue;
        }

        $account = user_load_by_name($existingUser);

        if (user_access('access administration pages',  $account)) {
            $newUsers[] = $existingUser;
            continue;
        }
    }

    $form['field_assign_to']['und']['#options'] = $newUsers;
}