I don't know if this is the absoloute easiest way to do it, as it requires some knowledge of PHP and access to a MySQL database. You can find simple "users online" counters on hotscripts.com in the PHP Scripts section. Set one of these up, and then you could just pass the user count to the SWF file in the URL.
For example, if you have the PHP variable $num_guests set to the number of guests currently on your site and $num_members set to the number of logged in users, you could pass these two variables to Flash in the SWF source URL like this: (put this between your two <object></object> tags)
<? echo "<param name='Src' value=\"usercounter.swf?members=$num_members&guest s=$num_guests\">"; ?>
Then, in your flash file, make two dynamic text fields and assign them to the vars 'members' and 'guests' respectively. Hope this clears something up. Let me know if you need any more help.