If you happen to have Huscarls, Marksmen or Swadian Knights in your prisoners, you'd better recruit them.
Camp menu --> Take an action --> Recruit some of your prisoners to your party
Then the game randomly selects a stack of prisoners and randomly decides if they join you or not.
If yes, then you lose 3 morale points per prisoner (the whole stack is recruited).
If no, then you cannot try to recruit any prisoner before the next 24 hours.
Edit : Actually if the recruiting succeeds or not, you can't try again before the next 24 hours.