It is a common feature in most of the sites that it is nothing but forums.All of them create a site alone for that  like www.forums.examplesite.com
But why don't you add it like a page for free!
It is very simple you have to just create a page then go to edit html and paste the code.
Where can I get the code!
The code for free forums is at http://www.nabble.com/
There you can see a option start a free forum
After all the steps you can see this

So you can add this code and enjoy forum facility for your blog for free!!
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