Hi smallbiz
I'm sorry for this lateness, somehow I missed this topic.
At this moment, there is no way to filter this 500 users collection. For the new registration, you can consider to create a new Joomla User Group and set it as default group for new registered users.
Next, you go to menu item of Jomsocial pages and look for 'Access' option and assign it to the new created user group above.
Regards