// delete old visitors
$timeout = time() - 60 * 5; // 5 minutes
mysql_query("DELETE * FROM sessions WHERE lastvisit < $timeout");

// update user
mysql_query("REPLACE INTO sessions SET lastvisit = '" . time() . "'");

// query users
$thisdomainquery = mysql_query("SELECT COUNT(*) AS totalusers FROM sessions");
$thisdomain = mssql_fetch_array($thisdomainquery);

// will output users on this site
echo $thisdomain['totalusers']; 

// pull info from other sites
// this will pull the info from the other domains with each new line as an array element
$network = file('http://www.anotherdomain.com/online.php'); 
$networktotal += $network[0];

// add the above two lines for all the sites you wish