If there is enough people how about hiring a bus? There are bus companies which do winery tours, I'll give one a call and find out the price if people want.
We went on a tour with Canberra Winery Tours last weekend, and it was well organised, with good communication from the Carol at the company. It included several wineries (McKellar Ridge, Vitner's Daughter, Clonakilla, Shaw's, Granitevale), and lunch at the new restaurant at Shaw's. We were in a mini bus with another couple, but they also had a larger 20odd seater for a hens party that was following along behind us. We received a voucher for a wedding gift, so I don't know what the cost was. They picked us up and dropped us off at home.
I think they can put together custom tours, and would certainly recommend them. Harvyk - their phone number is 6299 9898.