Add categories for new users on a multisite

Hi there!

I am looking for a solution here for adding categories for new users. The site is a blog portal that allows users to sign up and have their own blog subdomain. I need to be able to give these users the option to choose 1 of a number of "blog categories" when they sign up. Meaning, I need it to be a part of their user profile as soon as they sign up.

I doubt there is a plugin for this type of functionality so I am wondering if anyone can give me some guidance towards solving this myself? Which tables should I modify in the DB and what functions should I modify in my functions folder?

Any help would be greatly appreciated to get me off in the right direction.