Changing a Player's permission group

I am having a hard time understanding the Permission service and was wondering if anyone can point to the right direction where I could change a Player/Subject’s permission group?

Take a look at

I tryed this method, but i am using PEX for perms, and i making a loop on this method:

Map<String, Boolean> perms = player.getTransientSubjectData().getPermissions(SubjectData.GLOBAL_CONTEXT);
 for (String perm:perms.keySet()){ i test the permission...

And this is giving me a empty list. I tyed with only SubjectData too.