Support referral codes for users to measure recruitment effort

In another thread it was stated that no good way to track those who recruit/refer others to the site. I searched but did not see any reference to referral codes, similar to those used by and others, where a unique, static alphanumeric code (>6 digits) is generated in order to track those recruited to the site.

It’s not perfect, but I sent out several hundred postcard invitations to neighbors (if you recruit some, you get more invites) and got 18 confirmed. I did see people from my area who obviously got a postcard and signed up, but didn’t use the reference code, but I’m guessing it’s less than 25%.