In preschool, "working in groups" means splattering washable paint on each other, and struggling to share the dinosaur when you really, really want it for yourself. In kindergarten, it means, or at least starts to mean, assisting one another toward a shared goal.
OK, so your 5 year old probably won't be writing up task distributions with his teammates, but he or she will be in the early stages of understanding that working as a team doesn't mean you get less. When done right, it can get a job done better, with more giggles and a whole lot faster -- and that can mean early recess.
With a little (well, a lot) of practice, kindergarten teamwork can lay the groundwork for successful interpersonal interactions, especially in the workplace, for the rest of your child's life.
Next, the incredible world of numerals ...