Hi Alex thanks for your replay,
When I signed up for WPMU I assumed that Membership plugin has customizable registration form like many other membership plugins in the market. Anyway I am familiar with php coding but I am not professional. it would be great if you can give me more details like where to begin what part of database I need to modify? how does registration form work and how can I use the signup_extra_fields hooks etc... if you can provide me some sample code with comments that would be sweet...