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.