Most of the time people will follow comments in a hub where they had commented. So all you need to be concerned about is to post your own comment right after theirs as a reply. HP will automatically send them an email with a link to your reply comment.
You can also check to see how many people are following by looking at the follow/unfollow tab. You'll see that this tab includes the number of followers in parenthesis. If you are following your own hub the tab will be displayed as "unfollow" - meaning you can click it to stop following. You'll see this tab in every other Hub as well. and you can click "follow" so you yourself will be notified when someone replies to your comment in their hub.
Not to complicate matters, but there is a setting under "Email Settings" when you edit your profile where you can specify if you want to automatically follow any hub where you post a comment.