Groups
A shared feed that a small group of friends can post into: a place for a dinner club, a roommate house, or a trip to plan in one spot. This is distinct from the Lists you'll already have at launch for post targeting. Those live on your device and pick who sees a post. A shared-feed Group is its own space, with its own invites and members.
Why not at launch
Lists at launch already answer "who sees this post." Shared-feed Groups add member management, group-level invites, and a separate key-rotation story, all of which we want to get right before it goes live.
What it needs
Same as a regular post: your photo library when you attach an image. Nothing new at the device level.
How privacy works
Posts inside a group are encrypted for that group only. Only members can read them. When someone leaves or is removed, the keys roll so they can't read what's posted after that.