Unfortunately, group calendaring features such as that are not supported on the iPhone. I hope that Apple steps up to the plate and delivers a solution to this, otherwise the iPhone will not be as accepted in the business world where everyone is accustomed to this through MS Exchange (they're also accustomed to Exchange crashing).
Leopard Server is supposed to provide an opensource solution to the Exchange problem. Hopefully, they support their own technology and provide this kind of functionality on the iPhone.