This is a common question. It is also one that is not easily answered. A lot depends on the strength of the IT Manager and hisher relationship with other managers. The short answer is that if it is connected to the network or is supplied by IT, then IT should co-ordinate all activities related to it. This isn’t to say that, as in the case of your example, that HR should not be involved. This should be a co-ordinated team effort. IT should at least be on hand to monitor the activity of any third party related to anything IT. Don’t make this a difficult thing, but rather approach it as a team exercise as IT has the knowledge to understand things from the vendours perspective and can help to prevent others from getting “taken to the cleaners”.
Maybe on the other hand, you don’t want to be involved. I would suggest that you get involved and stay that way as IT is ultimately responsible for everything attached to the network.