List of Groups

ok, I figured it out by myself.
Basically going into the functions i got into get_group_tree() and get_group_tree() functions and starting from that i created this:

function load_checkbox_field_choices( $field ) {

    // reset choices
    global $wpdb;
    $group_table = _groups_get_tablename( 'group' );
    $root_groups = $wpdb->get_results( "SELECT group_id FROM $group_table WHERE parent_id IS NULL" );
    $group = array();
    foreach($root_groups as $gruppo){
        $grouped = Groups_Group::read( $gruppo->group_id );
        $group[$grouped->group_id] = $grouped->name;
    }

    $field['choices'] = $group;

    // return the field
    return $field;

}

I hope i helped someone.
If anyone has a more “good looking” code I’m gladly to see it.