Validate $recipients (only user objects are allowed, not uid's)

Excellent that works perfectly! Thank you for the quick response and explanation...

What I am trying to do is integrate Privatemsg so that when a user requests to join a group (og module) the group owner will receive a private message alerting them of the request.

I am using the latest dev release for Privatemsg (2009-Mar-06). At first I just dropped $admins in for the recipients and it was sending messages to "Anonymous".

$recipients = array(user_load($node->uid)); works

$admins sends to "Anonymous"

Here is the code from the og module beginning around line 980:

<?php
function og_subscribe_user($gid, $account, $request = NULL) {
  // moderated groups must approve all members (selective=1)
  $node = node_load($gid);
  switch ($node->og_selective) {
    case OG_MODERATED:
      $admins = array();
      og_save_subscription($gid, $account->uid, array('is_active' => 0));
      $sql = og_list_users_sql(1, 1, NULL);
      $res = db_query($sql, $node->nid);
      $admins = array();
      while ($row = db_fetch_object($res)) {
        $admins[] = $row->uid;
      }
      if (!empty($admins)) {
        // Prepend user's request text with standard cruft. Should be a
        // variable but that's a bit annoying.
        if ($request) {
          $request = t("\n\nPersonal message from @name:\n------------------\n\n@request", array('@name' => $account->name, '@request' => $request));
        }
		
        $variables = array(
          '@group' => $node->title,
          '@username' => $account->name,
          '!approve_url' => url("og/approve/$node->nid/$account->uid", array('absolute' => TRUE)),
          '!group_url' => url("og/users/$node->nid", array('absolute' => TRUE)),
          '@request' => $request,
        );
        
        $message = array(
          'subject' => _og_mail_text('og_request_user_subject', $variables),
          'body' => _og_mail_text('og_request_user_body', $variables) . $request
        );
		
		// integrate privatemsg
		privatemsg_new_thread('Group Request', $request, $admins, $author = NULL);

        // Send notifications to each admin; Sending an array of recipients
        // implies that this is a bulk message.
        module_invoke_all('og', 'user request', $gid, $admins, $message);
      }
      $return_value = array('type' => 'approval',
                            'message' => t('Membership request to the %group group awaits approval by an administrator.', array('%group' => $node->title)));
?>