Hey,
Log in backend and go to component > add user frontend > options
Now click on the tab permissions. You will see all acces settings there.
Now for example if you want to let authors and higher add users you mist click on author. You will see the permissionfields for authors.
Set Acces component and Create User From Front End to allowed and click on the save button.
From now on authors and above may acces and add users from the frontend.
IT IS NOT POSSIBLE TO SET PUBLIC ACCES FOR ADD USER FRONTEND. We never want strangers to add users to our website!
If you want to let users register and add your own fields to the form you must use OCR for this:
http://www.ghanja.be/en/downloads/components/one-click-registration/details.html