Modify do_action( 'register_post')

I fairly new to WordPress but do know programming. I am trying to update the registration and login processes to include more one more field of data inserted and retreived. Basically just adding an expiration date to a user.

I’m don’t seem to be able to find the file(s) that do the login and resitration processes. I need to hook both the login and registration. The registration to grab a value from another table before inserting the new user and the login to check the exipration date has not passed before logging them in.

Any help would be greatly appreciated. I just don’t seem to be able to find the right files for these processes.

Thanks!