Who is addressed by the term "some people" in Boyne's The Boy in the Striped Pajamas?

The answer to your question is that Mother often refers to the superiors in the Nazi government (and sometimes specifically Hitler) as "some people."  Mother uses this particular term to express her dissatisfaction with how Bruno's family is being treated (and specifically how the family is being ordered around).  There are a couple of quotations that prove this. 

We don’t have the luxury of thinking. ... Some people make all the decisions for us. We will just have to make the best of a bad situation.

This quotation contains a candid moment between Mother and Bruno.  Bruno has expressed his unhappiness about the new living situation.  Mother obviously agrees.  Bruno has just told mother that he "thinks" that they should simply move back home to Berlin.  Mother then makes the comment above.  Here mother admits it is a "bad situation" living near Auschwitz and that "thinking" is a dangerous activity in the Nazi regime.  Only the Nazis with real power are allowed to "make all the decisions."

We should never have let the Fury come to dinner ... some people and their determination to get ahead!

Here we have the proof that "some people" can possibly be in reference to Hitler himself.  "The Fury" is Bruno's mispronunciation of "the Führer."  It is only after Hitler invites himself to dinner that the family is moved away from Berlin.  In Chapter 5, Bruno shares the memory of Mother's words expressing her disapproval.