I'm not completely sure I understand. GPOs aren't really designed to be applied on a per-user basis, and GPO processing can become extremely inefficient when you start applying to individual users and groups.
You're saying you have an existing GPO and you want it to apply to only specified users or user groups?
OK, slow your roll, there. One post is sufficient.
If you want to use PowerShell to modify a GPO, you can do that. You'll need the RSAT, on Windows 7 or later (preferably Windows 8 or later), and there's a Group Policy module. You can run Update-Help after installing the module to make sure its instructions are installed.