What are some examples of animals who return to their group when threatened? Based on my (limited) understanding, some types of animals use the power of numbers for protection. I assume, then, some species instinctively respond to threat by immediately seeking out and joining their group/hive/pack, not just to hide or get lost within the group (eg., school of fish), but to also take advantage of the power of the organized group (eg., swarm of bees). Is this correct? If so, what are some examples? [Note: It is my understanding that bees don't flee to their hive in response to threat, but rather sting and release a pheromone to draw the other bees to the area and create a swarm.]

The specific description you have made is almost the definition of mobbing behaviour. Some animal species protect each other and especially their offspring by cooperatively attacking and harassing a predator. Individuals can't cope with the predator alone, but by working together they increase the chance of discouraging the predator.

African buffaloes, bison, gulls show mobbing behaviour.

Wikipedia article

The Royal Society for the Protection of Birds article