When designing a new system here's what I usually do (in my team).
1. Open up a Github Pull request with initial pseudo code.
2. All discussion about the architecture of the system goes into that pull request.
3. https://www.draw.io/ is used in order to draw the data flow in the system, how things communicate and wired together.
Often times, we have something we call a "grooming" session where all developers that are involved will meet and discuss the system, brainstorm and iron things out.
Architecting a system is more often than not a communications between people issue not just communication between software moving parts so laying out a foundation where everyone discusses and puts their input is crucial.