Where can I get an invite code?

Invite codes are only sent in the mail. If you don't have one or you misplaced your code don't worry - you don't need an invite code to join your neighborhood. Having an invite code just makes it a little faster to join.

Follow the steps at https://eneighbors.zendesk.com/entries/24077153-How-to-sign-up to join without an invite code.