The above are correct, if you want to make very accurate dry material measurements, there is no real substitute for weight. However, I don't see any way that you're likely to make a serious enough error on a priming sugar measurement by using volume measurements. Realistically, if you're 10% high or low, unless you've really controlled every other variable well and you are extremely particular about your carbonation level, I doubt you'd ever notice. Certainly you're not going to create bombs or un-carbonated beer due to using volume instead of weight.
3/4 of a cup is about right for a typical carbonation.