Final answer:
The command to join an existing swarm as a 'worker' node is docker swarm join.
Step-by-step explanation:
The command to join an existing swarm as a 'worker' node is docker swarm join. Option B, docker swarm join, is the correct command to use. By running this command on a worker node, it will connect to the swarm and join as a worker, allowing it to perform tasks assigned by the swarm manager.