$gsfh = $app->make('\Concrete\Core\Form\Service\Widget\GroupSelector');
echo $gsfh->selectGroup($field, $group = null, $noneText = null); //Build the HTML to be placed in a page to choose a group using a select box.
echo $gsfh->selectGroupWithTree($field, $group = null); //Build the HTML to be placed in a page to choose a group using a tree view.
//\Concrete\src\Form\Service\Widget\GroupSelector.php
Form (group selector) helper