Steve / Frozen,
Gougle groups is very limited indeed. To imitate a little of the sub-forums of the old forum there is the 'tag' option. When clicking on a tag, only that subjects are shown.
As you are an active member: I can make you manager of the group as well. That gives you the possibility to click on your (and others) name on the Group Member list and let it show the postings of yourself.
If you want that, until the original forum is back online, I can give you the admin rights.
Bar