to swarm; to be teeming (with), to bustle; to be bustling (with activity)
Tu ferais mieux de te grouiller.
You'd better hurry up.
Vous feriez mieux de vous grouiller.
You'd better hurry up.
Si tu ne veux pas louper le train, tu ferais mieux de te grouiller !
If you don’t want to miss the train, you’d better hurry up!