How to Add Extra field

[Saintless]

I have facing some problem. I need to modify registration page, when a new user will register he will give refer name, if refer name is present and refer person is a social group moderator then new user will be a member of that group but if refer is not a member of social group then a new social group will be create with that refer name and new user will be member of that social group. How that could be possible, any idea or reference please.

Thanks in advance