Polish collaborated in the holocaust, but how many collaborated in the actual conquering of Poland itself?
A decent percentage of the population of Szczuczyn, for example. Jews were Polish citizens just as much as non-Jewish Poles. So Poles who killed their fellow Polish citizens not at the urging of the Nazis, but because they wanted to achieve the same political aims as the Nazis, very much assisted in the invasion. The Nazis could focus their military energy elsewhere because of the ample assistance provided by some Poles, although it should be noted that the small number of Nazis still present in the town during the massacre actually stepped in to put an end to it, before following the lead of the Poles and murdering the rest of the Jews a few months later.
If a group of Americans went around killing a few hundred thousand independence-minded Americans during the Revolutionary War, it would be safe to say they were helping the British. As with the Poles who killed Polish Jews when the Germans invaded.