You've not used $mybb in global. Try this;
function checked_already($pid,$input)
{
global $db, $mybb;
if ($mybb->user['usergroup'] != "4")
{
error_no_permission();
}
$query = $db->simple_select("users", "username", "uid='{$input}' OR username='{$input}'");
$user = $db->fetch_array($query);
if (!$user['username'])
{
echo "Nothing found!!";
exit;
}
}