Sadly, there are no hangouts right now for creators of icon themes and GTK themes for Ubuntu. There are a few awesome icon themes and GTK themes which a lot of people are using. Creators maintain their own independent faces, either through social media or their own websites or even Github for issues and support.
I doubt whether there are any official resources or documentation to help them at all. Nothing is there ( that I know of ) which would encourage new talents to contribute in this area.