PrimeGrid is currently looking for a volunteer developer to build, test and maintain Mac apps. The current developer of PrimeGrid (and other applications that are used in the mathematics/prime search world) is no longer able to help us maintain the applications, which means that we may need to drop support for Macs over time (no such changes are planned right now or in the immediate future).
If you are able to help us, please don't hesitate to get in touch.
Moreover, the Berkeley Open Infrastructure for Network Computing (BOINC) open source project, which is the software infrastructure used by PrimeGrid and many other volunteer distributed computing projects, is also looking for volunteers to develop and maintain the BOINC client on Macintosh. The BOINC Client and Manager are C++ cross-platform code supporting MS Windows, Mac, Linux, and several other operating systems. BOINC currently has a number of volunteer developers supporting Windows and Linux, but their main Mac developer is winding down his involvement after many years. He is prepared to help a few new Mac developers get up to speed.
If you have Mac development experience and are interested in volunteering time to help support and maintain the BOINC Mac client please have a look at the more detailed description here: https://boinc.berkeley.edu/trac/wiki/MacDeveloper
If you are not a Mac developer, but have other skills and are interested in contributing to BOINC, the link above also has more general information.