In AWoIaF, you can recruit various sellsword companies or parties that roam the map, or kill them to remove opposition. Some of these will loot and raid villages.
In order to recruit them, you must engage a conversation with them and choose to hire them. If your relationship with their faction is too low, they will not agree to your terms and attack instead. You must have space for their entire army to join you and sufficient money in order to recruit them. Because of need for space, choosing to talk to them can pose quite a risk if you have a negative relationship as you'll often need to engage with a smaller army.
They can be found near Lorath
The Stormcrows are 502 unit large troops entirely comprised of Veteran Freeriders and 2 unique units. Their leader is Prendahl na Ghezn.
They can be found near Myr and costs 100,000 to hire.
The Moonbrothers are approximately 200 unit large comprised of hill tribesmen faction units. They are of the minor faction Hill Tribes.
They can be found near Gates of the Moon and often roaming towards Moat Cailin
They can be found in the North
The Second Sons are 503 unit large troops entirely comprised of Veteran Freeriders and 3 unique units. Their leader is Mero.
They can be found near Volantis and costs 120,000 to recruit.
The unsullied is a unique company that's available for hire but do not roam the map like the others or aligned to any faction. They are an elite spearmen which can be upgraded to Veteran Unsullied.
The unsullied and veteran unsullied have zero cost upkeep once your purchase them. Upgrading still costs a significant sum.
A man in the inn of Qohor sells Unsullied for 75,000 for 50 units at a time. A week of game time must be passed before you have a chance to purchase them again.