Best practices when adding single sign-on to your app with the Live SDK:
Your users will stay connected across all of their Windows 8 devices.
Your users get a single sign-in experience that is consistent with Windows 8 and their other apps.
Creating the sign-in experience within your app is easy; you can do it with as little as 3 lines of code.
Read more here: Best practices when adding single sign-on to your app with the Live SDK - Windows 8 app developer blog - Site Home - MSDN Blogs