You can find the neighbourhood code on the invitation letter. The ambassador of your neighbourhood has put this in your mailbox. If you haven't received a letter, you can send a request to the ambassador to get you accepted in the neighbourhoud, after you registered on Hoplr. In this case you don't need a code.
Are you already a member and you would like to invite a neighbour? Then you can simply print your personal invitation letter on the website (https://www.hoplr.com/invite/offline). The neighbourhood code can be found on your letter. Be discrete about your neighbourhood code towards people who don't live in it.
More ways to your invite neighbours? Go to How do I invite friends or neighbours?