$gid = current($form['group_ids']['#value']);
$node = node_load($gid);

$form['massadd'] = array(
'#type' => 'select',
'#title' => t('Select students to add to the group'),
'#required' => TRUE,
'#multiple' => TRUE,
);

$users = entity_load('user');
 foreach ($users as $uid => $account) {
  if ($uid && !in_array($uid, $exclude)) {
    $form['massadd']['#options'][$account->uid] = $account->name; // tried realname instead
  }
}

...

 $form['submit']['#weight'] = 0;
  // We validate our own way.
  unset($form['#validate']);
  $form['#submit'][] = 'opigno_simple_ui_form_og_massadd_massadd_form_add_roles';
  $form['#submit'][] = 'opigno_simple_ui_form_og_massadd_massadd_form_add_state';
  $form['#submit'] = array_merge(array('opigno_simple_ui_form_og_massadd_massadd_form_prepare'), $form['#submit']);

$query = db_select('realname','rn');
$query->join('users','usr','usr.uid=rn.uid');
$query->fields('rn', array('uid'));
$query->condition('rn.realname',$mail,'=');
$result =$query->execute()->fetchAll();

if (!empty($result)) {
$uids =array($result[0]->uid);
$account = user_load_multiple($uids);
$account = array_shift($account);
}