Исполнимый создатель Python 3.0.1

Я бы попробовал это:

// prepare an empty $result collection that will store the ids of the users
$result = collect();

// probably a number, not a string, in your database: we should keep the right type
$id = 5;
addRelatedUsersToResult($id, &$result);

dd($result);

// add all the related users ids to the result 
// and call itself for each realted user (to check their relations)
function addRelatedUsersToResult($id, &$result) {
    $relatedUsers = Users::where('pic_for', $id)->get();

    // add the related users and checks their children
    foreach ($relatedUsers as $relatedUser) {
        $result->push($relatedUser->id);
        addRelatedUsersToResult($relatedUser->id, $result);
    }    
}