All SIM cards are free to receive. If you send a SIM card (through your recruiter page) to a friend and they activate it then you will earn 500 points (£5) which is added to your monthly payback total and your friend will get £5 free credit deposited in their account as Airtime Credit.
This is the link to your sim recruiter page - https://giffgaff.com/orders/mgm
There are no other discounts when you activate a SIM card.
Do I get a discount if I buy a second payg sim
Not a discount, but you can get £5 bonus credit on the new account when you activate a SIM from your own SIM order page: https://giffgaff.com/orders/mgm for use in a different device.
You will also receive 500 payback points (£5 value) on your existing account during the month after the new SIM is activated (provided you have recruited less than 15 new members)