Press C, or use the comment button, then click anywhere on the canvas or inside a frame. A comment on a frame follows it when you move it.
Reply in threads and resolve them from the canvas menu.
Edit or delete your own comments.
Type @ to mention a teammate. A dot shows when someone mentioned you in a canvas.
Comments from teammates appear, change and disappear live.
On a frame, Comment here pins a comment at the playhead; it shows on the timeline and a click jumps there.
Write @feega in a comment and feega works on the node you pinned it to, then replies in the thread. This spends credits like any generation (Unverified: billing of comment-triggered runs).