note: in this response the word "department" is meant as a non-osTicket term, but an organizational unit in your organization.
I imagine that you would have to setup a Organization for each department.
Add all members of the department to the Organization.
And then setup autocollaboration for that Organization (and make the manager the primary contact).
note: this is not something that I have ever had to setup, but I imagine that it should work okay for new tickets. It would not retroactively grant access to tickets that already exist prior to this feature being turned on.