Hi, here is my idea:
1/ Blue middle area - this is representing totally private area.
2/ Now, limited sharing is presented by two blue circles, connected by lines. Since we set up visible boundaries (by circles and inner parts of them), this visually leads to "limits", yet we're still connected to user.
3/ To show "openness/full community", there are another connected lines with circle heads, spreading outwards. Notice, they are not limited by any boundaries, they're connected to user/users by visual intersections.
(This comment references Entry #19)