The Signup Box widget allows people that have viewed your site to become members.
When you drag the widget to where you want it to be on your site, an edit window will appear. It lets you edit the title and description of the signup box and what is written on the button that the visitor needs to click to join.
Once the visitor has clicked the signup button, they can sign up with an email address, Facebook, Twitter or LinkedIn. You can set which of these options you want to provide in the admin center, under Setup --> Advanced.