Thank you Steelflex for that very helpful comment.
Anyway Rom, it could be a bug, but try this first:
If you have any other NPCs around you, fire up the party screen and see if you can add them. (You should have 3 columns if you do have NPCs there you can add to your party.) If you can, then you should be able to add your prisoner as well. If you can't, then maybe something's missing.