Some sites may want to offer visitors the opportunity to get updates via email. for example a School Website may want parents to be able to signup to get the latest news.
There is not built in option to do this in Glow Blogs. You can use a third party service such as mailchimp.com to provide this. You can add a link to the signup form to your blogs sidebar.
Mailchimp has a lot of features for communication with blog readers, you may prefer a simpler service: Feedburner, a google service will send out updates via your blogs RSS feed to subscribers. You need a google account to set up on Feedburner.