The social group is where you can only post one after the other, when you have enough members it goes to a social group forum. The social group forums are at the bottom of the page. I'm presuming it is just like the forums here where anyone (in your group) can start a thread and reply too. Only people in your group can actually see the forum so it is completely private. The only difference from these boards is that YOU are the moderator of your own forum!