You can confirm your address with the unique neighbourhood code that's on the invitation letter. The code is only distributed to local residents and ensures that outsiders can't access the platform.
You only need the code once, when registering. If you need a code to invite a neighbour on a later moment, you can find it here https://www.hoplr.com/invite/offline.
If you have not received a code you can still complete your registration. You will get access as soon as the ambassador of your neighbourhood verifies your address. If you do not want to wait for the approval of the ambassador, you can send an e-mail to firstname.lastname@example.org with a picture of a letterhead or other proof of residence in attachment.