Have a look at WebLoginPE here
http://modxcms.com/WebLoginPE-1.3.1-2142.html depending on what you actually need to do here of course.
Use MODx, or the cat gets it!
It is a 2 step process:
There is the managing of the login needs a snippet for the front-end interaction (you can use a snippet for registration as well or do it via the manager). WebLoginPE and WebLogin are the login management snippets available (PE has more features).
The second part of the equation is setting the web user group and assign it to a document group then assign the "private" document to the same document group.
I just took a look at websignup’s code and I don’t think it sends a message to the admin as well. It would be easy to add this by editing the weblogin.common.php file and adding a second mail() call but with the admin as the recipient.
Actually, thinking a bit more, you could use a plug-in to do this (so no hacking needed) by setting it to the OnWebSaveUser event (some one please confirm).