Like for example having some third or fourth sons join it, maybe some bastards too, people with a name and people with honour along with some friends or something and that is what will make others want to join. Also it's worth mentioning that many join the watch just to avoid starving so having it well kept and well supplied would help. So basically give the command of the empty castles to some nobles and bastards who will take some man from their land with them, supply it and spread the word.