Under the blank canvas, there's a chat area where you can talk to the other users to decide what to draw or how to do it. In terms of the included tools, you can find a series of adjustable brushes with a huge number of settings that let you draw practically anything you can think of.
With the possibility of adding loads of layers, you can use as many as you need, in addition to adding or modifying the changes made by other people. Lastly, you can record your changes and then watch them later or show them to the rest of your collaborators. This is a very useful option when your friends want to know how you made certain changes.