They will get a letter from Member Services about it. The first referral in a calendar year is $200 I think (too lazy to go look it up on the website!). But in any case, they will get to choose between a gift card, credit against their dues, maybe one other choice. Additional referrals in a given year are worth more, on a sliding scale.
PS - they can look it up on the DVC website also - it's under Sharing the Magic I think. It will list any referrals and whatever the member has coming / has received for bonuses.
We used a referral when we bought and it took them FOREVER to get their $200. I had to call & email our guide several times. When I say forever I'm gonna say like 6 months! Just don't let your guide forget!!
MY DS joined the end of October and I received an e-mail this past Thursday asking me what referral bonus I wanted. I signed in at the DVC website and selected my choice. If you take the Disney gift card or the Visa card, the message says it can take up to 7 weeks to receive it. You can use your referral bonus towards dues, the Disney gift card or Visa card or towards the purchase of additional points. First referral is $200, second is $350.