[Admin]

Just having $activeusers in a template doesn't mean it will actually show something.

You need to put the code for it first.
This is taken from index.php:

$activeusers = "";
$loggedinusers = "";
if ($displayloggedin) {
  $datecut=time()-$cookietimeout;

  $loggedins=$DB_site->query_first("SELECT COUNT(*) AS sessions FROM session WHERE userid=0 AND lastactivity>$datecut");
  $numberguest=$loggedins['sessions'];

  $numbervisible=0;
  $numberregistered=0;

  $loggedins=$DB_site->query("SELECT DISTINCT session.userid,username,invisible,usergroupid
                              FROM session
                              LEFT JOIN user ON (user.userid=session.userid)
                              WHERE session.userid>0 AND session.lastactivity>$datecut
                              ORDER BY invisible ASC, username ASC");
  if ($loggedin=$DB_site->fetch_array($loggedins)) {
    $numberregistered++;
    if ($loggedin['invisible']==0 or $bbuserinfo['usergroupid']==6) {
      $numbervisible++;
      $userid = $loggedin['userid'];
      if ($loggedin['invisible'] == 1) { // Invisible User but show to Admin
        $invisibleuser = '*';
      } else {
        $invisibleuser = '';
      }
      if ($loggedin['usergroupid'] == 6 and $highlightadmin) {
      	$username = "<b><i>$loggedin[username]</i></b>";
      } else if (($mod["$userid"] or $loggedin['usergroupid'] == 5) and $highlightadmin) {
      	$username = "<b>$loggedin[username]</b>";
      } else {
	    $username = $loggedin['username'];
	  }
      eval("\$activeusers = \"".gettemplate('forumhome_loggedinuser')."\";");
    }

    while ($loggedin=$DB_site->fetch_array($loggedins)) {
      $numberregistered++;
      $invisibleuser = '';
      if ($loggedin['invisible']==1 and $bbuserinfo['usergroupid']!=6) {
        continue;
      }
      $numbervisible++;
      $userid=$loggedin['userid'];
      if ($loggedin['invisible'] == 1) { // Invisible User but show to Admin
        $invisibleuser = '*';
      }
      if ($loggedin['usergroupid'] == 6 and $highlightadmin) {
	    $username = "<b><i>$loggedin[username]</i></b>";
	  } else if (($mod["$userid"] or $loggedin['usergroupid'] == 5) and $highlightadmin) {
	   	$username = "<b>$loggedin[username]</b>";
	  } else {
	    $username = $loggedin['username'];
	  }
      eval("\$activeusers .= \", ".gettemplate('forumhome_loggedinuser')."\";");
    }
  }
  $DB_site->free_result($loggedins);

  $totalonline=$numberregistered+$numberguest;
  $numberinvisible=$numberregistered-$numbervisible;

  $maxusers=explode(" ", gettemplate('maxloggedin',0,0));
  if ((int)$maxusers[0] <= $totalonline) {
    $time = time();
    $maxloggedin = "$totalonline " . $time;
    $DB_site->query("UPDATE template SET template='$maxloggedin' WHERE title='maxloggedin'");
    $maxusers[0] = $totalonline;
    $maxusers[1] = $time;
  }
  $recordusers = $maxusers[0];
  $recorddate = vbdate($dateformat,$maxusers[1]);
  $recordtime = vbdate($timeformat,$maxusers[1]);
  eval("\$loggedinusers = \"".gettemplate('forumhome_loggedinusers')."\";");
}

So just stick this in membersonline.php, right after requiring global.php.