1. You can set up email subscriptions by using this guide > http://en.support.wordpress.com/widgets/follow-blog-widget/

2. If you would like a follow button to appear on the bottom right hand corner of your site for those who do not have WordPress.com accounts see here >
http://en.blog.wordpress.com/2011/09/21/more-traffic-for-your-blog-with-the-follow-button/

3. You can set up a customized contact form http://en.support.wordpress.com/contact-form/ on a static page http://en.support.wordpress.com/pages/ or in a text widget http://en.support.wordpress.com/widgets/text-widget/ and above that form you can ask those who wish to have their email address "captured" to use it.

Note: Your site stats page displays how many followers you have, who your followers are, as well as how many people are subscribed to post comments on your site.

Dashboard > Site stats > Total, 'Followers' , Shares
http://en.support.wordpress.com/stats/#totals-followers-shares

Click "blog" and when that page opens click "email followers"
http://wordpress.com/#!/my-stats/?blog_subscribers&type=email