If you have not created an account yet, you can use your friend's referral code. Important: You must use the referral code at registration.
When you're creating your account, there will be a button to add a referral above the orange Sign Up button. Click that and add your friend's referral code in the field, and then click "Add Code".
Then, just complete the account registration and your friend will now have you as a referral!
I created my account already and forgot to use my friend's referral code. Can I still use it?
Unfortunately, once your account is created, the referral code is no longer usable.