This is separate code, but you could put it wherever you wanted:
global $db_prefix,$modSettings;
$today = strtotime("24 hours ago");
date('j') == 1 ? $thismonth = $today : $thismonth = strtotime(date('F') . ' 1');
date('l') == 'Sunday' ? $thisweek = $today : $thisweek = strtotime('last Sunday');
date('M') == 'January' ? $thisyear = $thismonth : $thisyear = strtotime('January 1');
$query = db_query("SELECT
COUNT(dateRegistered > $thisyear OR NULL) as year,
COUNT(dateRegistered > $thismonth OR NULL) as month,
COUNT(dateRegistered > $thisweek OR NULL) as week,
COUNT(dateRegistered > $today OR NULL) as today
FROM {$db_prefix}members
WHERE is_activated = 1", __FILE__, __LINE__);
$row = mysql_fetch_assoc($query);
settype($row['today'], "integer");
settype($row['week'], "integer");
settype($row['month'], "integer");
echo 'Total Members: ', $modSettings['totalMembers'],'<br />';
echo 'New This Month: ', $row['month'],'<br />';
echo 'New This Week: ', $row['week'],'<br />';
echo 'New Today: ', $row['today'];
It also returns the number for the year. I figured I'd go ahead and get the data just in case someone might want it.