Before Davening, it is permitted to drink water. Tea and coffee as well are permitted, but only if it is to help one concentrate better while Davening. To add sugar or milk is dependent on whether one needs it to drink the coffee or tea, or if it is just to make it sweeter. In either case, those who drink should not do so together in a group. So too, it is best to say Birchas Hashachar before you drink.