We strongly prefer to engage users via GitHub. This automatically preserves useful information for posterity and provides a natural way to link to specific code.
Please create GitHub issues to request help of any kind, including questions about the project.
If you prefer to communicate wtih the PRK community via email, use parallel-research-kernels@googlegroups.com. This list includes dozens of people who have an interest in the project, not just the developers. It is a good forum for asking general questions, the answers to which may be of interest to many users.
If GitHub issues does not work for you, you can email the developers directly. If all else fails, you can find these in the Google Group history (see above).