This is the concept: When Invitation system is enabled the regular registration will be disabled (members can only register by getting an invitation). A new member (and existing members as well) have an initial number of invitations which can be used to invite friends to the forum. New Invitations will be automatically granted when a specific thing have been achieved ( For example: based on number of new likes, new posts, new threads) which can also be configured from the user upgrades menu.
This option is disabled by default. It will make the registration system of xenForo more flexible and configurable and will help to better control new registrations. This feature is scalable and many options can be added in the future.
Like this post if you want this to be implemented in xenForo.