In the comments are missing fields for existing users

I would make sure that users who have already left comments and have registered, they can log in by entering your username and password for the comments.
How should I do?

For example, as you can see here: http://www.seojoomla.it/seo/web-marketing/trucchi-seo-for-the-small-business.html#comments
there are no fields.
thanks a lot

stefano