Every group has its own leader. The themes of the working groups may differ very much, from theoretical (where the focus is on the mathematical and physical side of astronomy and astrophysics) to practical (where the focus on observational astronomy and the use of telescopes cannot be ignored). This is just a rough explanation, as there have also been groups that fit neither of the two explanations but focus more on background, like ones with a more philosophical character. The theme of the group depends very much on the leader attached to it and his/her background.