Somehow, Sign In as another user option is no longer available. I remember they still have it in the beta release. This is weird, I don’t see an advantage or improvement by removing that option. Maybe its to improve the security? By forcing users to re-login to Windows using the user account.
Whatever.. The fix is found below and shared by Nick in this post. http://nickgrattan.wordpress.com/2012/07/23/sign-in-as-different-user-and-sharepoint-2013/
You will have to update the Welcome.ascx file with the following entry before the element ID “ID_RequestAccess”
After the update, the Sign In as Different User Option will appear.