Here are a list of Group buying websites:

http://www.groupola.com/
http://www.kgbdeals.co.uk/
http://likebees.com/
http://www.vivavoucher.com/
http://www.mypiggyback.com

From Techcrunch:

"For those who aren’t familiar with the group buying model, here’s how Groupola.com works:

Each day, the site will feature a deal in each of the eight cities it is launching in. Then if enough people commit, the company offering it will honour the deal, which will be anything from 50-90% off the full price. If not enough people sign up to that particular deal, the offer is withdrawn and no money passes hands.

Examples include, savings at restaurants, spa days, beauty and hairdressing, experience days, sport and leisure activities, theatre tickets, and concerts."

http://eu.techcrunch.com/2010/02/24/gro ... in-the-uk/

Clever idea, I like it!