You can follow Drawpile development over at GitHub, but you do not have to be a programmer to participate! Some ways to help include:
The preferred way to make bug reports and feature requests is to use the GitHub issue tracker.
A good bug report includes a clear description of the bug and the steps needed to reproduce it. The latter is not always possible, as some bugs can appear very sporadically, but any description will help. A bug that cannot be reliably reproduced is very hard to fix! A session recording that demostrates the bug would be immensely helpful, but this only possible for certain types of bugs.
An example of a bad bug report is: "It doesn't work." When I get a message like this, I always have to respond with standard questions like: "What doesn't work? How doesn't it work? What sort of error messages are you seeing?" etc. So please provide that extra context in your first message and you'll get a more useful response sooner.
When requesting features, you should include a description of the feature (what it should do) and a use case (what it's for and how it should be used.) A misunderstood request might get rejected outright or implemented in a manner the you did not intend.
Are you interested in developing Drawpile? Drop me a mail and I'll help you get started :) I'm especially looking for help with Windows and OSX development (e.g. help with platform integration and fixing platform specific bugs), since I primarily work in Linux myself.
To get the development environment set up, clone the Drawpile repository from Github and install the dependencies. The Building from sources wiki page has instructions on how to do this on Linux, OSX and Windows.
For debugging tablet related problems, take a look at QTabletTest, which is a simple program for checking if tablet events are working.