Instructions:Found this link while browsing the website, sorry if already posted.
What is Recommend a Friend?
Recommend us to friends, relatives or work colleagues and we will send you and the people you recommend Play.com discount vouchers. You can refer as many people as you like (up to a maximum of 25 per day). For every person who creates an account, they will receive a discount voucher and then when we post them their first purchase, so will you.