I think tags might be the best way if categories are too resource intensive. How about this:
- You have one ‘hidden’ category for ‘group posts’ (won’t show up on the normal indexes/lists, etc.)
- Each custom group has one tag made for it, that only gets used in this category
- Creating a topic in the group will create a topic in the hidden category with the group tag
- Viewing the group topics will show all the topics with the tag, just a different page layout or whatever.
Could be a start to achieving this?