You don't use them in a site. You use a site to get people to subscribe and then send them newsletters by email.
To get subscribers you'll need to offer valuable & rich content, the best to start with your existing customers.
If you're looking for a a newsletter provider try the one below. I am not connected with the site other than I use it myself, they cost $19 for 6 months and handling everything for you, all you do is create a newsletter and send it via the site. http://www.yourmailinglistprovider.com/
Websites generally send newsletters to their existing customers or subscribers to let them know regarding their new services or product information...