once you on giffgaff you can invite your frineds using https://giffgaff.com/spread and you will get 5 pound cash back per activated sim card in December and your friends will get 5 pound credit after first top up.
a ) For a " referral " SIM, the minimum payment is £10. Referral SIMs ( with a £5 credit bonus ) are received from an existing member's " spread giffgaff page ", forum banner or other personalised link.
b ) For a " free " SIM, the minimum payment is £5. Free SIMs are ordered from the giffgaff main page. Please note that free SIMs do not have a credit bonus.