When the bees start to hang out the front of the beehive in large numbers and there are a lot of bees under the lid when you inspect the hive, it is time to add a super. Remember to place a queen excluder between the new super and the brood chamber below. This is done in order to prevent the queen laying eggs in the super, which is where you want the bees to store the honey.
If you do not add a super then you run the risk of the bees becoming so overcrowded that they will swarm. This phenomenon involves the queen leaving with half of the bees in the hive. The remaining bees will try to build emergency queen bee cells in order to rear a new queen for the colony. Therefore, while swarming does not necessarily mean your hive will cease to function, it however it does mean that you will not get as much honey from the beehive that year.