The "conversations" functionality is a different thing. It's basically an instant messenger to chat with other members in private. Basically like the normal messages but on every page. The group stream works as a stand-alone conversation option which lets you have conversations in public, or group only, it does not show in the conversations tab. In addition, you can create discussions within the group that's what this is. It's a bit confusing for me as well. I think it is possible to show the group discussions on every page, but I have to look into this.