public static void addUserToFreeGroup(final User user) throws IllegalArgumentException {
    if (paidGroup.contains(user)) {
      throw new IllegalArgumentException("User already member of paid group.");
    } else {
      if (!freeGroup.contains(user)) {
        freeGroup.add(user);
      }
    }
  }
@Test
  void testAddUserToFreeGroup() {
    var user = new User("Free User");
    UserGroup.addUserToFreeGroup(user);
    assertFalse(UserGroup.isPaid(user));
  }