QMCPACK is fully open source, and contributions of any size are very welcome.
Guidelines and examples of different ways to help are given in the QMCPACK manual https://qmcpack.readthedocs.io/en/develop/introduction.html#contributing-to-qmcpack . We aim to make contributing straightforward with lightweight policies typical of any mid-sized project. Contributions to the manual are described at https://qmcpack.readthedocs.io/en/develop/contributing.html -- even the smallest typo fixes are welcome.
Please open an Issue if you have any questions on features, roadmap, tutorials, installation, possible bugs etc. If you are already running QMCPACK, provide as much detail as possible in the report (e.g. copies of Nexus and QMCPACK input files.)
If you like the project but don't have time to contribute, you can still support it by adding a star on GitHub or by citing QMCPACK when appropriate.
Thanks for your interest in improving QMCPACK.