Here's how it works
On your Channels page, you'll see a new blue "Add Group" button for any Facebook channel you've added.
If you click it, you'll be taken to a page that looks like this:
From your list of available Groups on the left, just click each one you want to add, and it'll appear in the list on the right. Easy peasy.
Now if you return to your Channels page, you'll see the new Group(s). These Groups will be available everywhere your other Channels are: for posts, feeds, stats, etc.
Here's an example of a post I just did to the "Social Media Hub" Facebook Group:
Kane Miller from DoSocial
Kane has a background in computer engineering, but now focuses on creating new and interesting web services. His passion for social media and marketing encouraged him to create DoSocial.