Well, I'd say that first off if you can wait on Joe to put this change into the core, that would be the best option. But if you can't wait and really want to use the custom user handler method, you'll need to either do some custom development in C# in Visual Studio 2010 (or work with someone who can), and do the following:
- Create a user handler class, starting from Joe's provided code skeleton, and build it to a DLL file.
- Deploy that custom DLL to the /bin directory of your mojoPortal installation
- Create a .config file as outlined on that user handler page and place it as instructed.
- Run www.yoursite.com/setup to register the user handler with your installation
Now, that's the ideal bare minimum required. Realistically, there will be a lot of testing cycles needed to make sure that it's doing what you want, before you deploy to production. Also, I'm assuming that you would be hardcoding the role name directly into the user handler code. If you want to make a user-maintainable screen for selecting the roles, that adds an additional layer of complexity. I made ours user maintainable by creating an aspx page that does per-site database updates, and added it as a custom Administration menu item.
I hope that helps,
Jamie