How can I associate an advanced forum with an organic group?
I have struggled with this problem myself for some time and finally decided to just create a separate content type for my group forum topics and "create" a forum in each group with those topics.

I created a new content type called "Group Forum Topic." In Advanced Forum settings, I added this content type to the "node types to style" so that it would make it look like the forum.

On the og_content view, I added an additional block and named it "Group Forum topics" and I changed it to use the forum topic list format. Using the view from Advanced Forum (Topic list) as a template, I set up the view to display the "group forum topic" content like a forum.

This allowed me better control over the permissions -- I am using entityreference-prepopulate to prepopulate the group when they add the content and I have the group content set to private, so I know that the content will be kept private to the group. 

I tried using other modules, but ran into issues with auto-selecting both the forum name and the group and didn't want to chance the user accidentally not selecting one of those things on their own. After a lot of testing and trying, this seemed to be the easiest solution to having a private group forum on the group page.

Keep in mind this does NOT create a listing for the forum on the forum index page. What I have done instead is add my group listings to my forum index page view, so that beneath my "regular" forums, I have a listing of "groups," which contain their private forums inside.

This is my first answer here and I know this question is old... but I ran across it when I was researching this issue myself and thought perhaps the next person looking for a resolution might find it and it might help them!