You only need to add water to straight antifreeze but if the antifreeze you buy is the 50/50 antifreeze mixture then no you don't need to add any water to it.
Although it costs a bit more per jug the 50/50 mix antifreeze is the way to go when adding antifreeze to your cars coolant system because it means that antifreeze will be mixed correctly which is important to keep your cars cooling system working properly and prevent the engine from freezing during the winter months.