Myhabit offers $25 just for 5 friends that register via your invitation link (no purchase required). Max credit per customer is $50. This offers is valid through January 2, 2013. Credit will be delivered via email between Jan. 11 and Jan. 15, 2013.
The credit will be valid for 6 months from the date it appears in your account. You may not use referral credits to buy gift cards or to pay shipping, handling, taxes or any other applicable transaction costs. Offer is not available to Myhabit members located in the following states: Arkansas, Colorado, Illinois, North Carolina, Rhode Island, or Connecticut. You may not use spam to advertise Myhabit.
See full terms and conditions on the Myhabit web-site.
Note that many other flash sale sites (private shopping clubs) pay for invitations, but most of them give you credits for customers, not for free registrations.
Myhabit is an american private shopping club powered by Amazon. It offers international shipping to select countries and accepts international debit/credit cards as well as Amazon gift certificates.
Off note: Fab and Homesav flash sale sites also give you credits when your friends register (no purchase required).