http://modxcms.com/creating-a-web-user-group-and-document-group.html
On this page, it is explaining how to set up a web user account. But after the first step, you are supposed to leave the TOOLS main tab and go to SECURITY.
Then you can find a different USER subtab and continue with the instructions. I am new to MODx and took me a while to find that.
-william