Docs say:
> In Differential and Maniphest, you can mention another user by writing:
> @username
> When you submit your comment, this will add them as a CC on the revision or task if they aren't already CC'd.
I think that for consistency if you create a task and mention a user in the description, he should be CC'd as well when the task is created. We wondered about this today while test-driving Maniphest on our first real-world project.