It’s been noted that the “Sign in as a Different User” menu command is missing in SharePoint 2013
One suggestion for a fix can be found here: http://www.little.org/blog/2012/07/17/launch-your-web-browser-as-another-user/.
This “Sign in as Different User” menu item is very useful when testing applications, but it can lead to problems especially when opening documents, say in Microsoft Word. So, it may be for these reasons that the option has been removed in SharePoint 2013.
You can add the menu item back in, but I would suggest only doing this on test or development SharePoint servers. To do this, repeat this edit on all servers in your SharePoint farm:
- Locate the file \15\TEMPLATE\CONTROLTEMPLATES\Welcome.ascx and open in a text editor.
- Add the following element before the existing element with the id of “ID_RequestAccess”:
- Save the file.
Now, the menu item shall be displayed:
Update 16-Jul-2013: As noted by a number authors (and correctly so), modifying files in the hive is not generally good practice. It’s presented here as a quick (and dirty!) solution. Look through the comments against this post for some other solutions that you might want to try too.