Runrun.it is integrated to Gmail and Microsoft mail. That means anyone who has a Google account (Gmail or company email on Google Apps) or Microsoft can register or login Runrun.it.
Runrun.it users can associate any Google/Microsoft account. Just click on your name on the right top menu, select “My profile” and click on "Link your Google/Microsoft Account." Every time you login into your Google/Microsoft account in a browser, will automatically be logged in Runrun.it.