How to add a new user role

Hi,

I want to add a new user role in my database!

I already added the structure in my registration page.

This is a screenshot of my registration page:

I want know how to save this value of radio button in my database (user or usermeta table) and check if seller’s reference is valid (I already have a table of seller reference in my database)

Any help will be appreciated and thanks in advance!