In the U.S., one cup contains 8 fluid ounces.
Dry measure, 1 cup = 8 ounces.
Liquid measure, 1 cup = 8 liquid ounces
It doesn't really matter what you're measuring, the only thing is after it's cooked, what is the true weight. I'm sure you will loose a little weight on the ground beef after it's cooked. (water, oil, fat)