Take a look at the WebLoginPE snippet:
http://modxcms.com/extras/package/495
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With WebLoginPE, you can specify an extra table to keep custom data in, and the fields for that custom table. The registration form contains the fields, and the data is inserted into the custom table. It has a built-in user profile editing system, and a management system so admins can work with the users and their profiles. It also has a built-in user listing and profile viewing system. I’m not sure if you can easily modify it to allow grouping like you want, but it would be easy enough to write a snippet to do that, since we know where the data is. It sounds like you want some AJAX stirred in as well, though. Still, most of the work is already done for you with a stock WebLoginPE.