You can register Silverstack and LiveGrade once on one machine and afterwards make the license available to all users.
After the activation just move the files in “~/Library/Application Support/Pomfort/Licenses” to “/Library/Application Support/Pomfort/Licenses” (You will have to create the folders Pomfort/Licenses).
Please Note: Since OS X Lion the Library folder in your home directory is hidden. In Finder use Command-Shift-G (⌘⇧G) and enter ~/Library to open the folder.
When restarting the application it will load the licenses from the system disk and every user should be able to use it. With that solution multiple users of this machine can use the Pomfort product license.