Some interesting news today, even if we're not entirely clear on the exact way these work (either Serebii explained it really poorly, or he doesn't have any more of a clue than the rest of us). So there's this item called the Majin Lamp that, when used in a dungeon (is it consumable? Who knows?), summons Hoopa in a form chosen at random. Extra manpower is nice, but the real kicker is that if you then speak to Hoopa, he can summon a random legendary. Problem is, sometimes this legendary will fight for you... or it will fight you. So it's a real double-edged sword. Serebii then clarified in the forums that this does NOT replace traditional legendary recruitment, and you can recruit legendaries in the normal ways from previous games. It's not known whether you can only use the Majin Lamp when your party isn't full, and other such considerations regarding party size.
Also, this thing that looks like the affinity chart from Xenoblade serves the purpose of... replacing the bulletin boards. What the fuck I didn't expect this. So apparently, the way it works is that when you select a Pokémon, they'll give you a mission, and upon completing that mission you may recruit them, as well as unlock other Pokémon that can give you missions, and so on. The more missions you do, the more missions you unlock. It's unknown whether it's the only way of recruiting Pokémon (which would REALLY suck), and whether the number of missions you can do is finite, etc. ...we really don't know much about this, so the wise thing to do is to withhold judgement. We do know legendaries can be recruited through this, though.