Subscribers are the people who will be notified about the post once you submit it.
Responders are the people who are assigned to (and notified about) the post and responsible for taking action on a post. When you are assigned to a post as a responder, you will see the assignment show up on your Dashboard.
Subscribing to a post
You can subscribe someone to any post at any time by clicking "Change" link at the bottom of any post.
Assigning to a post
You can assign someone to a post by click on the "+" sign on any post in the lower left corner and then selecting the person you want to assign from the popup window. See below.