Sonderkommando
Usually, the Nazi camp guards chose people for the Sonderkommando groups right after those people got to the concentration camps.  They almost always chose Jewish prisoners.  These people were told they would be killed if they did not agree.  They were not told what kind of work they would have to do.  Sometimes, the new Sonderkommando would find the bodies of their own families in the gas chambers.  They were not allowed to change jobs or refuse to work.  The only way they could stop working as Sonderkommando would be to kill themselves.