I wouldn't add them to the HHA unless you're planning on completely draining your Avios balance on this booking - it'll be a hassle/impossible to split up your assets again later.
You can add them as a 'Friend and Family' but then I don't think you can use their Avios for a booking.

As previously pointed out, the real challenge will be to find 4 reward seats available in one go, unless you're planning to travel WT?