For Post Production in the Compositor, I heavily rely on using Object/Material Index masks. Normally, I select a single object and then give it an unique number in the Pass Index/Material Index slider in the Object/Material tab. Using the ID mask node in the compositor usually gives a black/white image mask.
However, this doesn't seem to work on group instances. When grouping objects together and using the resulting instance, one can still give this object a Pass Index value. But the result isn't a black/white image, but rather a white/alpha file (but messed up), or the file is completely empty. Giving the individual objects of an instance a Pass Index doesn't work as well when using the instance itself.
Is there a way to get the Pass Index on instances to work?