I had originally posted:

$groupid = 12345;
$fb->add_post
    ->id($groupid)
    ->set_message('I like beer.')
    ->publish;
[download]

However, the method is named "to," and not "id". I apologize for the error. As you showed (and included for future searchers), the code should be:

$groupid = 12345;
$fb->add_post
    ->to($groupid)
    ->set_message('I like beer.')
    ->publish;
[download]

I would like to propose you look at Recipe 3 since it queries "me/accounts" to post as other accounts within your control.