The phpBB3 Invitation Only mod allows existing users on a forum to register new users they personally know by adding them through a new UCP module. This disables standard open registration and ensures only people known to current users can join. The new users receive an email with login details, and admins are notified of additions. A log tracks additions and custom fields capture details. The goal is to create a tightly-knit community where all users are connected through existing relationships.