Not to stifle further discussion, but /u/commiespaceinvader gave a good rundown of the issue and the law in question in their answer to the #GermanDeathCamps issue.

The Germans certainly did use Polish collaborators and there were instances of Poles supporting the German occupation government. Note there is a crucial distinction between supporting the occupation government and the occupation itself. Poles were fairly unique compared to countries like France or the Netherlands in that the German occupation authorities did not directly elicit Polish collaboration. Nonetheless, the needs of running even a truncated state like the General Government or even one ran on apartheid-levels of discrimination like the new Reichsgaus carved from Poland necessitated the Germans to rely upon Poles for some duties. The Blue Police, a force of collaborators made up of Poles (see this answer of theirs by /u/marisacoulter on the vagaries of the Blue Police and collaboration) were one such force. There were accounts of Blue Police being used in the round-ups prior to deportations in addition to the ghetto police. The Blue Police were also involved in various anti-Jewish actions outside the ghetto. There were other Poles who assisted round-ups of Jews and reported escaped Jews and received rewards from the occupation government.

But "voluntary" is something that needs to be taken very carefully in the context of German occupation. The occupation regime in the East was incredibly violent and there were all sorts of reminders for any collaborator what the price of non-collaboration was under the Germans. Harboring Jews could earn a Polish gentile a death penalty or prison sentence (which could be tantamount to the former) while the reward for turning Jews in could be what amounted to scraps of food. While there were some individuals who collaborated out of personal gain or ideological resonance with Nazism, collaboration in these regions could often be the case of being the least-worst alternative for more than a few.