Which tools e-commerce companies sign up their users well?

Signing up users is the most important step in building a loyal userbase. Ideally, you want them to buy products in your site, so that they can be a registered user as well a buyer. But, users usually don't buy in one session. So, it is important to sign them up without interrupting their shopping experience and re-engage them later. Based on this criteria, we have ranked the tools ecommerce companies in United States that do a good job in signing up users: