One pound of ground beef weighs 16 ounces, or exactly 453.59237 grams using the standard avoirdupois pound. For everyday use, that is about 454 grams.
The pound figure describes weight, not one fixed shape or volume. A package of ground beef can be a flat tray, a compact block, or a loose mass, while still weighing one pound. A raw pound can also weigh less after cooking as fat and moisture leave; cooked weight varies.