// production code:
Group::add(Element* e) {...};
Group::remove(Element* e) {...};
ElementDerived::ElementDerived(Group* group){ group->add(this);}
ElementDerived::~ElementDerived(Group* group){ group->remove(this);}

// test code:
struct Test_ElementDerived : public ::testing::Test {
  void SetUp() override {
    ElementDerived* p = nullptr;
    EXPECT_CALL(group, add(_)) // how to set expection on the pointer argument?
      .WillOnce(Invoke([&p](auto base_ptr) {
        p = static_cast<ElementDerived*>(base_ptr);
      }));
    sut = std::make_unique<ElementDerived>(&group);
  }
  void TearDown() override {
    EXPECT_CALL(group, remove(sut.get()));
  }
  MockGroup group{};
  std::unique_ptr<> sut;
};