Are you wanting to get a list of users that could register but don't? Or are you wanting to get a list of failed and successful logins to the SSPR Registration site?
If the former then you can do an XPath Query to accomplish that. The XPath query could then form the basis for a set. You could have an MPR to trigger a workflow to email people reminding them to register.
For the later IIS logging can be enabled.
David Lundell, Get your copy of FIM Best Practices Volume 1 http://blog.ilmbestpractices.com/2010/08/book-is-here-fim-best-practices-volume.html
- Proposed as answer by David Lundell -- FIM MVPMVP Tuesday, January 07, 2014 5:08 PM
You can achieve this through Search Scopes from FIM 2010 Default. I am sharing a link for the same as well :