Whenever I refer a friend to Giffgaff, it's always because they need a new phone and their existing deal has run out. New friends need new phones.
Because the friend wants to purchase a new phone immediately they do not want to wait for a Giffgaff SIM to put in their old phone. This means they end up signing up as a new member without anybody receiving a referral bonus (as they have to buy a goodybag with their purchase).
There should be a way to refer a friend so that they can buy a phone and join Giffgaff immediately.