Mistake is:

echo elgg_view('output/url', array(
        'text' => $group->name,
        'href' => $group->getURL(),
        'target' => '_blank',
        'container_guid' => $group->guid,
    ));

Another way:

$event_guid = (int) get_input('guid');
$event = get_entity($event_guid);

$group = elgg_get_page_owner_entity();

$container = $event->getContainerEntity();
if (elgg_instanceof($container, 'group')) {
        YOUR_CODE_HERE
}

Thanks for your reply.

Am sorry the code returned a blank page but when i remove this code
$container = $event->getContainerEntity();

The site will come up without any changes.

Make sure you have an event entity. Modify the code like this

$event = get_entity($event_guid);
if($event){
    $group = elgg_get_page_owner_entity();
    if (elgg_instanceof($group, 'group')) {
            YOUR_CODE_HERE
    }
} else {
    echo "no event found";
}    

Thanks for your contribution @Webgalli

I tried the code, yes i have the event entity,
but no changes took place. Still didn't display the group.

I don't know what must be wrong.