If you share a board with other people, you can edit it together in real-time.
This makes it really easy to join a live collaboration session (like a brainstorm or workshop). It also provides great ambient awareness of what other people are up to.
The live-sharing popup (triggered from in the top right of the screen) shows who's online in boards you share. Clicking on the name of the board will take you there.
When someone else is making changes to a board you're viewing, you'll see a blue circle with their initials attached to the element they're editing. If someone else is editing something, you won't be able to edit it until they're finished.