in the lead up to HoD they were talking about being able to control the armies of your dominions and puppets. Did that happen? How does it work? In my UK game I released Canada as a dominion. I don't seem to be able to move their armies around. We just fought a war against the US together and they helped a lot with occupying, like puppets always did, but I didn't see any option to 'take control of their armies'.
If you call them into a war, then a button will highlight in the diplomacy options for that country. Click on that and you will be able to control their units.