Adding a subscribe form to your website is a great way of getting more contacts that you can send emails to.
Go to your contact lists and under "Get more subscribers" you'll find the HTML form code that you can add to your website and style using CSS.
Go to the contact list you want to add to and under "Get more subscribers" you'll find a link to the subscribe page of that list.
Use the Minutemailer Wordpress plugin to easily add a subscribe form to your website. Download the plugin or install it from within Wordpress by searching for Minutemailer in the plugin menu.