Hello,
I am experienced javascript/html5 developer, and i have worked on similar projects in the past.
It is really important how much simpler is whiteboard that you want to implement, and do you want it written really from scratch. Also very important thing is which is the oldest browser that you want to provide support for.
Do you need login functionality developed, user accounts management, chat, etc?
Canvas solution would be probably harder to implement, since drawn data needs to be transferred to the server for multiple users. If we would go with canvas, idea would be to provide multiple layers for each individual user and sync them using base64 over ajax and combine them with other canvases. Potential problems may come up with "undo" functionality and stacking of drawn lines from various users.
Second solution would be to use svg based paths and shapes, which of course can be easily synced over ajax, and they should have wider browser support than canvas (i saw some colaborating tools using this technique). In this case every shape would be entity for itself, but using some permissions, users should be able just to manipulate shapes which they "own"
Please don't hesitate to contact me if you have further questions.
Best regards,
Vlatko