You can apply at the Incubator over here : http://incubator.duolingo.com/apply
thank you very much! I have just applied :)) I hope I will be accepted!
All the best! One more thing, when you want to reply to a user's comment you can click on the 'Reply' button that you can see below the comment. In this way that user will get notified and know that you have commented on his/her comment. :)