right now when you assign a Task to a user that has no access rights per the current Visible To/Editable By Policy, then view and edit are granted automatically to the user upon assignment.
A similar thing should happen when you CC someone. That user should be granted View rights on the task (this already includes the ability to comment).
Right now nothing happens, although it is implied by the Assign behaviour.
Since this will impose some performance cost (according to @epriestley), then it might be better to make this into an opt-in feature burried in Maniphests config?