DFTB+ is an open source program and you are very welcome to join us in maintaining, extending and improving it!
In order to participate you may
report issues, using the issue tracker,
resolve any of the open issues,
extend the documentation (especially DFTB+ Recipes, if you have a nice demonstration for any of the features of DFTB+)
add new functionality to the code,
integrate DFTB+ (either as library or as standalone) into other software packages.
The development is coordinated via the public repositories on GitHub. If you plan to implement a new feature in DFTB+, you may consider to contact us first, so that we can discuss about the feature and coordinate your efforts with the ongoing development.