I'm sorry, that's not possible on WordPress.com. You can learn more on how to invite people to your blog at http://en.support.wordpress.com/adding-users/#invite-people-to-follow-your-blog
Also, adding a Follow Blog Widget gives your visitors the opportunity to subscribe to your blog.
There are no restrictions if you setup a self-hosted WordPress.org blog instead of a WordPress.com blog, but there are some added responsibilities. We have hosting recommendations at the following link if you are interested.