Currently any external contribution will require explicit arrangements. More streamlined rules will be put here when this becomes possible.
The reason for this temporary mode is the fact that BindKraftJS is a core with potentially number of modules that can extend it - differently in different projects. Also the integration with the server is a matter that makes running it a bit difficult without deeper knowledge of the platform. The forthcoming features and example platform setups will solve these problems and enable anybody to familiarize with the basics even without additional help - and that is the moment when the contribution will take more typical shape.
When to expect that?
I cannot define exact time frame, but I hope that before the end of 2019 there will be enough introductory materials to make it possible for everybody to use BK over various servers and as a consequence form opinions, ideas for features, components and so on - everything needed in order to make the source code contribution possible without the need to be led personally though obscured plans, abstractions and principles.