It's not possible to create a user using Content on the Fly.

Steps to reproduce:
1. Login as admin
2. Click "Create" on the Dashboard
3. Select "User" as Content Type
4. Select "eZ Platform" as starting location

Expected:
User creation form is displayed

Actual:
Error occurs:

The option "intent" with value null is invalid. Accepted values are: "register", "create", "update".

UPDATE for 2.5.0:
Right now creation form is opened, but Username field is disabled.