Avogadro includes over 300 common molecules and molecular fragments to make building larger structures easy.
Under the “Build” menu, hold your cursor over “Insert”, and then select “Fragment…”.
A database of fragments will then pop up (shown below). You can filter the selection if need be.
After you’ve made your fragment selection, click “Insert”.
The fragment will be inserted and the Manipulate tool will be selected so you can move the new fragment around the window.