How does Adams show the importance of individual contribution to a group in Watership Down?
Each of the initial rabbits who leave the warren have specific skills to contribute. Fiver's original premonition leads them to escape the warren, which is gassed by humans, and he correctly predicts danger several more times throughout the books, saving the group. Hazel is able to convince others to follow his lead, and without his influence and decision-making skills, the group would break up. Bigwig is a large and strong rabbit, and several times he is the one to protect the others from an external threat. Blackberry is slightly smarter than the other rabbits (they have trouble with abstract reasoning and drawing conclusions from available information) and he is the one to realize that they could float on water if they stood on something else that floats; this information saves their lives later during the fight with Woundwort:
The Efrafan does had never seen a river and it would certainly have been beyond Pipkin or Hawkbit to explain to them that they were on a boat. They -- and nearly all the others -- had simply trusted Hazel and done as they were told.
(Adams, Watership Down, Google Books)
Adams shows how each rabbit brings an important skill to the group, and since the rabbits are smart enough to know when something works, they appreciate and encourage each skill when it is appropriate. In this manner, individual ability is shown as a vital part of teamwork; when the group can't make a decision because they have too many opinions, Hazel takes charge, and when Fiver predicts danger, they know to believe him.