When you’re creating a Group Policy Preference, you can use Environment Variables. A cool thing is that when you press F3, a window will open which will display all kind of Environment Variables you can use.
Wanna give it a try?

• Open Group Policy Management Editor
• Browse to [User Configuration], [Preferences], [Windows Settings], [Folders]
• Create a New Folder by right clicking in the Folders Section and choosing [New], [Folder]

   

• The New Folder Properties Dialog will spawn

  

• Select the “Path:” Section and Press F3, the [Select a Variable] dialog will spawn

   

Now you can select your variable.

TIP: when you want to use variables which are user dependent. Use [User Configuration] instead of [Computer Configuration].