It would be nice to have an option on the group page, perhaps below “members” that shows the private “categories” that this group has a membership for.
This would help with the case that I have users that are “afraid” to post to the private categories. This fear stems from a post process that starts from a “general” view of all categories. They don’t trust themselves to always select the private forum from the dropdown of all forums.
If they were already in the private category before posting, and the category defaults correctly, they would be okay as they assume starting a post from the private area keeps it private. And while yes, they can navigate to the forum manually to begin with, there is something about just picking out the private forum from a list of all forums that they are not comfortable with.
This method would provide a route for those users to navigate to the private forums via their group membership. By going through the group first, they essentially have qualified in their mind that they are in a private section. Any navigation within that area would keep them in the “private section” thus making them feel more confident with posting in those categories.
To make the process even more complete, adding the option to “hide” private categories from the front page would re-enforce the aforementioned mental model. If a user wanted to get to the group categories they would have to tunnel through the group first. Of course, going down this path would also potentially justify enhancing the group default page to be a “group” filtered version of the current home page.
And of course, bringing your group membership out to the main page completes the workflow.
Where selecting the group would show you a list of the groups you are part of with the links to dive in to those group pages.