Best place to start collaborating is via our #rpi4-uefi-dev channel on the Developer-Ecosystem Discord server. While not directly affiliated, you may be also interested in the #raspberrypi, #windows-on-arm and #works-on-arm channels.
As work is being done in the TianoCore upstream, the authoritative source location is the official edk2-platforms tree. https://edk2.groups.io/g/devel is the development mailing list where all upstream patches are posted. Staged/wip work is done by some developers in the PFTF branches.
The Pi 4 firmware work is a derivative/continuation of the earlier Pi 3B/3B+ UEFI Firmware effort. See RaspberryPiPkg for a historical tree, upstreamed into official edk2-platforms as of early 2019. As the Pi 3B/3B+ UEFI firmware enables 64-bit Windows for the Pi 3B/3B+, see Windows-on-Raspberry and Pi64.win for directions (and discussions) on deploying Windows 10 on Pi 3B/3B+.