Drawpile: Collaborative drawing
Just came across this poking around on getdeb.Drawpile
It's billed as an open source clone of opencanvas, is cross platform and comes with it's max users per canvas set to 20.
Also: Note the pressure sensitivity option.
Edit: After routing around, other noticeable features in the newer version: Disable guests from drawing, load an image to draw on top of, fully customizable shortcuts.
Oddly, the Windows version is only available at version 0.4.
Looks like you don't need the Qt libraries.Unfortunately, you need the Qt libraries on Windows and Mac (Ubuntu will install them automatically.).
Qt for Mac | Qt for Windows
Would people please try out the Mac and Windows version to see if they work as well as the Linux one. If all goes well, awesomeness abound.