Notification of completed task not only for the person who created and assigned the task

Hello everybody, I'd like to ask whether is it possible to set when a task is completed, the notification to go not only to the person who assigned the task, but to be able to set ( when the task is created) also other persons? My assistant helps me to create the task, but I would like to be notified  when the task is completed , automatically, as in case of the person who created and assigned a task.

Thanks for your help !