Communities functionality can be easily extended using SharePoint apps.
Since Communities are SharePoint sites in the background, community owners can add those apps by accessing the Site Contents page in SharePoint and adding an app there.
Newly added apps will then appear in the community menu and therefore accessible to all community users.
If owners want to change the order of the new option in the menu (or its behaviour) they can access the navigation option in the SharePoint Site settings page > Navigation.
