Okay. I was able to dig it out. It is part of their new features.
Here is a link to busy.org which contain all the latest features included is the Referral and it explain it more.
Here is a link
https://busy.org/@busy.org/busy-v2-4-release-new-features-that-will-make-you-love-busy-org
In summary whenever you want to invite someone, for instance http://busy.org/i/@yungchief and yours will http://busy.org/i/@ackza to invite others