Okay - usual disclaimers apply - I cannot be held responsible if your forums die, or Elvis comes knocking on your front door etc...
In admin/docreategroup.php find this
-----------------------------------------
Make sure we don't already have 99 groups
$query = "
SELECT COUNT(*)
FROM {$config['tbprefix']}Groups
";
$sth = $dbh -> do_query($query);
list($num) = $dbh -> fetch_array($sth);
$dbh -> finish_sth($sth);
if ($num == "99") {
$html -> not_right("{$ubbt_lang['MAX_GROUP']}",$Cat);
}
and change it to...
// -----------------------------------------
// Make sure we don't already have 99 groups
// $query = "
// SELECT COUNT(*)
// FROM {$config['tbprefix']}Groups
// ";
// $sth = $dbh -> do_query($query);
// list($num) = $dbh -> fetch_array($sth);
// $dbh -> finish_sth($sth);
//
// if ($num == "99") {
// $html -> not_right("{$ubbt_lang['MAX_GROUP']}",$Cat);
// }
That is it - if you want to see how it works on my site go to
http://www.nonleague.com register, select a team from the Ryman Premier League - go to Change Display settings under My Home and select another team in the Ryman Premier League, and see how the navigation above, and the side bar alters. (Only the Ryman Premier League is fully using the system at the moment.)
As I say, it may or may not work for you depending upon how you need to use groups.
I should also mention that my groups are not sequential - my numbering system goes up to over 5300, due to my database diving
at the moment - but expanding all the time....) 
