You can integrate Pressero with a free email service called mailchimp (mailchimp.com). Mailchimp has a form that you can add to your site that will automatically add them to your mailing list. They give you a script that you add to the head content (In Pressero admin you edit the head content in Sites > Settings > SEO/HTML tab.) You will see in Sites > Site Users that there is an area for integrating with mailchimp. *Please note that the script to add a sign up form will not work if you ever choose to add a SSL (security certificate) to your site to use one of our advanced payment methods. We have another option in that case: http://support.aleyant.com/kb/a31/mailchimp-sign-up-form-on-https-sites.aspx
If you want to have the sign up for another service besides MailChimp, you may find that they have some html that you could try adding to the head content of your site to add an newsletter sign up form to your site. If the script creates a <form> tag on the page then it will not work correctly. Most services have a way to generate a link that will go to a sign up form. That would be the best solution rather than putting the form directly on the page.
Here are two other ways to have a form on your site:
1) You can also create a form in Pressero and add the form to a page you create. To create a form, go to Preferences > Forms. To add the form to a page you've created, you would go to Sites > Site Name > Pages > Forms tab.
2) You can use a free form creator. We recently had a blog article about doing this. Here is a link to that blog article: http://blog.pressero.com/2012/12/14/using-a-form-in-your-web-to-print-storefronts-to-generate-sales-leads/