To be clear and to answer enterpriseJM's question, the whole question about add_user_to_blog('X', $user_id, 'XXroleXX');
It works fine for me just like this, for instance:
add_user_to_blog('11', $user_id, 'contributor');
No serialized anything. The only problem is that then the user gets added to that blog as their primary blog, as well as getting their own blog...
If you do need the serialized data, it's in the wp_usermeta table.