On a public blog, people have to choose to receive your post notifications by email by signing up for it. You do not add them, they add themselves.
Also, the site linked to your username is set to private so people cannot sign up for email updates.
If you are going to keep your site private, then you add users at settings > privacy and they will have to sign up for a wordpress.com username and password. You can add up to 35 for free. If you need more than that, they you would have to purchase the unlimited private users upgrade.