When you're in the editing mode, on the right side of the page where you are working are the kinds of capsules you can choose for your hub. So you simply need to choose "Comments" along with "Text", "Photos" or any other capsule types you want from those offered. You have the control of that.
As soon as you choose it, the Comment capsule will appear at the bottom and will stay at the bottom even if you add other hubs afterward.
Of course if others either don't know how to add Comments to their hubs - or choose to not add them, that is their option. Then as another replier mentioned, you can vote and rate them that way.
The comments capsule doesn't have any appreciable influence on ads on your hub. Don't sweat that.
You also have the control of what comments actually appear for others to see. You can not approve them when they come in or delete them if they've already been added. I've never had cause to disapprove or delete any, though. They can actually enhance the message of the hub. HP gives us the chance to exchange ideas and share our writing experience with our fellow-writers here. There is much value in that.