Okay - I needed, also, to get the total number of members. Your variable $numbermembers is likely empty. The difference between your code and mine, is that I needed to be able to pull this data from an external file (not part of vBulletin).

Including global.php alone, was not enough to have the userstats array set.
You must set the $specialtemplates array - in order for $vbulletin->userstats to be populated. The cache, is simply that this must be included BEFORE global.php is included/requred.

$specialtemplates = array(
'userstats'
);

Hope I was able to assist.