You can't pay them though, enemy units require enemy resource points to stay in your party.
That's why I hand them off to the nearest friendly group. If you see a friendly vs enemy battle, and the enemy is holding prisoners or is human or at least able to be taken prisoner, then join that battle (knock out a few enemies if you have to) and in the rescue prisoners/captured enemies screen leave the enemy troops you can't support behind as that will cause the friendly troops to claim them as they take whatever you leave.
Key is to start up a side's momentum. If you're evil, you should be trying to crush Gondor, then Rohan, etc. As good, you want to be supporting Dale and the Dwarves at the start, then hit up the other sides once you've knocked off Dol Guldur and the like.
Yeah but I'm afraid if I ride north first, there's not going to be a Gondor to come back to. Rohan is taking it on the chin pretty hard too. They went from Strong to Fair in less than a week. The northlands seem to be handling themselves pretty well for the moment. It's also a big adjustment after playing an evil character first. My party cap is around 44, whereas with my evil character, it can be upwards of 120 if I use mostly orcs.
In my game I started in the North. Gondor lost Caer Andros and Rohan was down to Fair and three cities before I rode south to start training them some reinforcements with a squad of 15 Mirkwood infantry and 5 Iron Hills dwarves. I only took so few so I could focus on local troops to hand off as a garrison at Rohan's remaining cities while still having a hardened backbone for my army.
By the time I had gone north again I was down to 1 Iron Hills dwarf and 12 Mirkwood infantry, but Rohan has garrisons with about twice as many men as before and many of them are veterans due to me having focused on training and leadership skills on the main character and giving all the companions I've recruited so far (5 companions) as much training skill as I can after they have the physical stats to manage to kill stuff now and again for more leveling.
Also I should add that Iron Hills Dwarves' final upgrade is really, really good but have high upkeep. They are called Gror's Guards, and the one that I still have is the only one who made it to that rank from Iron Hills Miner out of the five originals. They work very well in tandem with Mirkwood Swordsmen, as the Swordsmen are taller and carry shields and have better armor, while Gror's Guards have ridiculous power strike and a gigantic axe to chop orcs while the Swordsmen do some blocking of both arrows and melee (the Gror's Guard ax looks cool but I'd recommend changing the model's axehead to be slightly smaller)
Also, something interesting in my game is that I failed a quest for Erebor (dwarves) and now they are neutral to my wood elf character. That means I get the attack/disengage screen rather than dialogue for all their armies. At the same time though I can still do quests, which netted me a dwarven caravan to escort to Dale or something. However, since you finish the escort quest by talking to the caravan next to the destination, I can't finish that quest and had a caravan of dwarves following me until I outran them and orcs ate them.
One thing I learned though is that deliver caravan quests have no time limit. Effectively, you will have a caravan filled with reinforcements behind you, as long as you allow an enemy party to attack them (bonus are the rank and influence points you get for saving them). They also work wonderfully as a distraction when you want to run away from a Host. I recommend an elven caravan, with Mirkwood seeming to have the best equiptment.