Runrun.it is integrated to Gmail. That means anyone who has a Google account (Gmail or company email on Google Apps) can register or login Runrun.it.
The companies registered by a Google Account will be redirected to a page to import users. Runrun.it filters emails from the same company (if not a simple Gmail account) or add the top contacts, and suggests the register of these new accounts on Runrun.it.
Runrun.it users can associate any Google account. Just click on your name on the right top menu, select “My settings” and click on "Link your Google Account." Every time you login into your Google account in a browser, will automatically be logged in Runrun.it.