How much of an environmental impact does Google's buses have?

How many cars does Google's buses remove from the roads per day in the bay area?

Also, is a there any company that allows smaller startups to pool together to provide the same benefit to their employees?

Not sure about the environmental impact, but supposedly research shows that it increased the rental prices by up to 20%.

Source: http://io9.com/yes-the-google-bus-is-affecting-rental-prices-in-san-f-1510968923

There are indeed several providers providing shuttle services for smaller companies. PeterPanBus is one of them.