How do you allow logged in users to comment via Disqus without extra login?
This question is old, but for the sake of the completeness: you have to use Single Sign-On from DISQUS API. In the comment section, logged in users will see themselves connected to the DISQUS system with the cretentials from their Drupal account and will be able to comment directly with no additionnal login. The anonymous users will have to previously connect to DISQUS, Facebook, Twitter or Google before sending their comment.
Note that this question has already bee answered here.