Do runners have to advance on a ground ball?

That depends on whether a runner is forced to vacate a base when a ground ball is hit. If there is a runner on first base and a ground ball is hit, the runner is forced to run to second base because the batter is running to first base. If there is also a runner on second base, that runner is forced to run to third because the runner from first is running to second. If a runner is not forced to run, they do not have to. If there are runners on first base and third base and a ground ball is hit, the runner at first is forced to run to second because the batter is running to first. But the runner on third is not forced to run because no runner is running to third base from second base.