i need a user registration form that have a field unique for each user. I mean that the user can register and insert in a field a number , that nobady user can re-use this number to register into the site.
Then he can chose the user grout to register.
Currently there is no such fields in such field, other than username and useremail. So if you can use that number as username, then it will work.
Or if you are a developer you can develop a small plugin for it or you can hire use to create such field for you.