You would first want to ensure that you record the current time when a user logs in. You must ensure you have a column in your db to hold this value. If you use a int field to hold a Unix style timestamp you can use a DataAssist update server behavior to update the record of the user. For this last login column just bind it to the php time function like this:
<?php echo time(); ?>
To display users logged in today you would need to devise a recordset that will select all of the users where the time stamp value is greater than the current time minus 12 hours or less than the current time plus 12 hours. This will give you a 24 hour period. You can them limit the results returned by this query to fit the display area for this info.