Would be easy to tell you if I knew the table structure of ipb.. but I dont and it dosen't look like it's free anymore so i cant check
your query would look something like
<?
$connect=mysql_connect("localhost","mysql_username ","mysql_password");
$db=mysql_select_db("forum_database");
$query=mysql_query("SELECT user_id FROM users_table");
$total_users=mysql_num_rows($query);
Echo("We have a total of ".intval($total_users)." users in our community.");
mysql_free_result($query);
mysql_close($connect);
?>