they are the volunteer workers whose task it is to help others

they are the volunteer workers whose task it is to help others

要去掉it，不然后面句子有两个主语了whose task和it

they are volunteer workers whose task is to help others

不能用 不然后面这个定语从句就有两个主语了。
they are the volunteer workers whose task is to help others

坚决不能用it。你这里的it指的就是whose task，其实就是volunteer workers' task(志愿者的任务）。用了的话就重复了。

写成两个句子就是
they are volunteer workers and their task is to help others.
他们是志愿者，他们的任务就是帮助别人。