As ronin said above, make sure you switch someone to militia then right click on the passer by. Just make sure you have a room available with a bed set to prisoner mode (select the bed then click the icon in the bottom left), that way you'll be able to bring them back and place them in the room.
Once you've done that, select your prisoner and click the prisoner tab. Click the "recruit" cross to turn it into a tick, and select "talk to them" underneath. When the warden comes in to check on them, they will try and recruit the prisoner
Make sure you have someone set to warden in the overview tab though, otherwise nobody will go and chat to the prisoners!
Once you've done that, select your prisoner and click the prisoner tab. Click the "recruit" cross to turn it into a tick, and select "talk to them" underneath. When the warden comes in to check on them, they will try and recruit the prisoner
Make sure you have someone set to warden in the overview tab though, otherwise nobody will go and chat to the prisoners!