The PROMOTE TO ADMIN button appears on the command bar when viewing a list of users or a user record. This is new with the 2016 Spring 16/Service Pack 1 release.
This allows a user to be made a System Administrator without having to manually add the System Administrator security role. Clicking the button add the System Administrator role.
The ability to use this command is controlled by a privilege named "Promote user to Microsoft Dynamics CRM Administrator Role".
More to follow