EDIT: Matalogude developer included support for GroupPro into master branch. You can grab his addon from Matalogue.
Matalogue is great addon for managing materials.
In my fork I added support for working with Group instances (so it works with GroupPro). Added support for Group Instances = when selecting group instance object (empty with duplication set to group), Matalogue will now show materials for objects that are inside this group. Previously it would show ‘no material’ message.
Hopefully autors of those addon will agree to merge those changes into master branches! Keep the thumbs up.