Seems like this could be a bit dangerous. If someone's on the grid but either isn't at their machine or doesn't want to set ready forcing them could mean a car on the grid without a driver. There's a reason anyone (i.e. you don't need to be an admin) can remove drivers from a grid.
Better solution would be to get player status and use function like timeout/idle time to verify players who join to grid list and don't set ready status after defined time.
When defined time has expired, players who are not ready should be kicked by host from grid list.