Well, if you're the one giving the expeditionary forces, you simply need 1 diplomacy point, and then go to the diplomacy screen and click on the expeditionary force option - it'll show all your units (the OOB will be present so it shouldn't be too hard to find what you want). Then once you've picked the right unit, you hit accept, and your ally will either accept or deny your expeditionary force. I *assume* it's helpful if the unit is already in allied territory. Then it's out of your hands, they supply and reinforce it as if t were their own unit, however, it will maintain whatever leader you put on it. It's typically not all that useful unless you're a minor who wants to drop some units on a major to help out - but generally I'd assume it's suggested you maintain control of them because the AI can be prone to do odd things and you can actively help it out lol. Early on in Hoi3 and especially in some versions of Hoi2, your allies would *constantly* give you expeditionary forces whose pop ups and resource draining capacities served to annoy many a gamer - it's quite a lot rarer in TFH I've found.