Besides selling them to slaver 'merchants' in taverns, you can offer your prisoners to join your army when making camp. You can go to a slave trader in a town, and sell any prisoners that you have and make some money. Basically - Always capture the rude and nasty Lords and set the nice and honorable ones free. Those will pay you for your 'common' prisoners, i.e. You can find slavers and ransom brokers in taverns. Once you have taken prisoners, you may recruit them to your party.
