I'm pretty excited that I finally deployed the og_vocab module on this site. This enables each group to maintain its own classification system for its posts. It also offers a handy block called 'Group categories' which helps subscribers navigate by taxonomy term.
You can see the block on the right hand side of any group page if that group has actually created and used a group specific vocabulary. Also, notice that the display of terms at bottom of each post is now grouped by vocabulary. For an example of both of these changes, see http://groups.drupal.org/node/192.
In order to create a group specific vocabulary, you must be an admin of a group or a sitewide editor. If you qualify, you will see a new categories tab on the group home page which helps add/edit.
This site has a legacy free tagging vocabulary which I think will stay in place. It doesn't quite fit in that well anymore, but I'm not too bothered yet.
I will announce this on site homepage in a few days. Please feel free to use this new feature in your groups. Meanwhile, your feedback on this feature is welcome.

Comments
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So...I don't get it. I've run several large group sites, and I find it less useful than a large tagging vocabulary...where you can automatically see all posts with the same tag for all content that you are subscribed to.
I'm not against having it...I just haven't seen good use cases for this. Basecamp has similar per-project categories, and the Miscellaneous one is the one that gets used the most (or people randomly pick a category). Even if there are hundreds of posts, am I going to browse by "promotion"? Maybe this is easier for advanced searching?
Help me out here....
same as taxo
well, lets start with the need i am trying to solve. one runs a group about Ecommerce, for example. In the group, people are talking about all kinds of stuff. Over time, there is no easy way to find old conversations, or catch up on a topic of interest like 'payment' or 'shipping'. Group admins and group members get frustrated.
The group specific vocabs help the admin organize his content. It forces users to affiliate their posts into categories (assuming required vocab). Basically, these vocabs are useful for all the same reason that global vocabs are useful on a drupal site.
Err...why not just free tagging?
So, why not just show a block which displays all free tags used for that group?
Just seems like it REMOVES the benefits of free tags...where, hey, if someone in the Artist group talks about paying for music (and tags it ecommerce, paypal) then it will surface across the entire site.
Then I can just subscribe to tags for the entire site and catch it wherever it is.
Anyway, like I said before, I don't want to say "turn it off" or "it's bad"...just exploring the use cases here.
Speaking of which...can we get tagadelic installed?
data entry is key
that would be equivalent for display, but not for data entry. the beauty of non free tag is that you prod or reqire users to use standardized terms. i think that UI experience is different enough that you can end up more content tagged the way you want it tagged. anyway, there are use cases for both.
if you haven't already, check out the issue tracker at the project hosting part of http://code.google.com. it combines the best of free tagging with constrained vocabulary. it does so through a bad ass auto-complete/select list within a free text field. hard to explain. i can give you perms to my toy project there if desired.