I manage two internal teams (direct reports) and external contractors (matrix reports) in a relatively flat organization structure and casual workplace culture. It is common for employees to help teams outside their own as long as deadlines and priorities are coordinated properly.
My team already has a lot of work to do, but I have a fellow manager of an unrelated team who sometimes agrees on new tasks/projects and deadlines with my reports without ever consulting with me first. Often he gives them very interesting projects that my reports are enthusiastic about and start doing, but thereby feel less motivated to perform on their main processes. When I remind them of their priorities they get very defensive.
I think this fellow manager has a few great ideas, and he casually gets the green light by our common boss, but the way he tries to implement them interferes a lot with our existing workloads and it would be better for my team and morale if he spoke to me first so we can coordinate schedules and timeframes rather than him going directly to my reports.
I once told him very casually "Hey, man, could you CC me in those e-mails about the Project Y you were talking about?" He said "Oh yes, sure! I'll send you an invitation to our next meeting..." but never got back.
I don't want to assume anything negative, but one of the things that should be clear to the fellow manager is that the team is supposed to do nothing unless it comes through me for planning. How can I communicate and enforce this without giving the impression that I am trying to stifle teamwork or the company's flat and informal culture of matrix cooperative work?