shared canvas and coordinate capture and storage.

This is what I need to achieve:

1. Client A places a red dot somewhere on her canvas. This dot is displayed on her canvas and the Canvas of Client B.
2. Meanwhile, Client B places a green dot somewhere on his canvas. This dot also appears on the canvas on Client A.
3. The coordinates of Client A's and Client B's dots are stored on the server.
4. Repeat from 1, thus adding more dots and storing coordinates.

Can someone please help me out here? This basic code will be a great help for me, and I am sure many others on here too.

Thank you, and have a pleasant evening.