Technically, you have little more than a
. You could rewrite the few additional member functions you wrote as global functions. Then you could take advantage of the full functionality of std::vector rather than just the very limited selection you happened to provide in your Group of Groups implementation. And you can take full advantage of the high efficiency of that implementation, rather than wondering if your own implementation is suboptimal.
But to answer your question, I've noticed the following that you should fix, simply because it's a common coding convention. If you override an index operator for your class, then you should supply two variants: one should be declared const and return a copy of the required element, the other should not be const, and return a reference to that element, so that it can be modified from outside:
const Group operator(int) const;
If you do this, it indirectly answers your question: in your current implementation, getIten() has the exact same functionality as the index operator (except for the range check). Therefore you should provide two versions: one a const function returning a copy, and the other non-const returning a reference.
Note that this answer is based on common coding conventions and has little to do with performance. Although of course, following common coding conventions is typically helps to obtain good performance, too. ;-)