The cheapest Virgin Blue fares are the ones on the Internet, and you can book up to 20 people using the online booking engine. The downside is that using the internet is an instant purchase, which may not suit. Group booking on virgin blue through the contact centre have the normal $15 each way levy which accounts for some of the increase in price (maybe available fare buckets also had an effect).
Qantas also has a minimum of 10 passengers to be a group (but you can't do it online)