You can only recruit "real" trainers; For example, the ones from the GTS. There was an event called "Magneton trade" where everybody on the GTS offered Magnetons, by seeking Magnetons, so that they get more people to the Join Avenue. NPC in-game cannot be recruited, but if we trade over the GTS or somehow else, you can either Invite me, Recommend to me a shop or Recruit me.
But the less your Avenue's rank is, the lesser are your chance to Recruit someone. I have a rank 33 Avenue, and cannot recruit; I tried 5 times, and they said:
Hmm... Sorry, I'm not interested.
So you need a high rank Avenue in order to recruit people.
Source: My Experience and my brain ♫ :)