We aren't changing any privacy settings with a bait and switch approach but in fact every group that someone is part of is something they either were invited to and joined explicitly or started themselves and invited others. every group is still private and you only know about the groups that you are in.
As more of these groups are formed we aren't exposing what groups you are in to other people joining. In fact other people joining don't know who else is on the platform unless they are in a group with them.
So this social graph that is being built isn't based on friend requests that have little inherent value but on the different groups you are part of that are based on specific relationships.
So what's your definition of "social graph"? In what way is "my list of my group subscriptions" a social graph? Is there a bigger graph? Who has access to that?