If you want to create custom register page then you use in wp-users Table
wp_create_user();
It will create the user and if you want to add custom field then save that field data in wp-usermeta table using
update_user_meta();
and last if you want to add role to the user then use
$u = new WP_User( $user_ID);
$role_of_user= $u->roles[0];
$u->remove_role($role_of_user);
$u->add_role('your_role');