Well, I think the title speaks for itself. I've seen a couple of people doing this on their comics but I can't find the way, link or button to do it myself. Or is it a "subscribers only" feature?

Go to the control panel for your comic, and click on Forum/Blog. Fill out the little boxes there to make a subsection for your forum, and then click "view forum" - voila! You now have a forum you can use and post in. You can also go back and make more subforums if you want. Also, if you want people to even know your forum exists, you have to put in a link manually on your main comic page. There's no automatic link that goes up or anything.