public static void addUserToPaidGroup(final User user) throws IllegalArgumentException {
    if (freeGroup.contains(user)) {
      throw new IllegalArgumentException("User already member of free group.");
    } else {
      if (!paidGroup.contains(user)) {
        paidGroup.add(user);
      }
    }
  }
@Test
  void testAddUserToPaidGroup() {
    var user = new User("Paid User");
    UserGroup.addUserToPaidGroup(user);
    assertTrue(UserGroup.isPaid(user));
  }