<?php
$friends = $facebook->api_client->fql_query("SELECT uid FROM user WHERE has_added_app=1 and uid IN (SELECT uid2 FROM friend WHERE uid1 = $user)");

foreach($friends as $friend) {
  $users[] = $friend["uid"];
}
$users_list = implode(",", $users);
?>

<fb:request-form
action="http://apps.facebook.com/YOURAPP/" 
method="POST" invite="true" type="YOURAPP Title"
content="Lets play YOURAPP today! <fb:req-choice url='http://apps.facebook.com/YOURAPP/' label='Confirm' />" 
>

<fb:multi-friend-selector
showborder="false" 
max="20"
actiontext="Invite your friends to use YOURAPP. " 
exclude_ids = <?php echo "{$users_list}"; ?> 
/>

</fb:request-form>