Apparently you attach all grounds to some common component, like the chassis and then you have a zero volt reference. As long as it is stable and the relative voltages to it are consistent, that should suffice for electrical systems. Handling overall charge of the craft might be a bigger challenge, but no idea how that would be handled.
Protective actions are required by the docking standard so it’s taken care of at docking but I find it very hard to find detailed information about implementations. It’s either very niche or considered sensitive enough to not be published widely.
I guess they use the same solution than during EVA which is running the Plasma Contactor Units to dissipate any buildup.
The docking spacecraft must follow a NASA standard[1] to avoid building up electrostatic charge.
From the Commercial Crew requirements document[2]:
3.9.3.13 Integrated Space Vehicle Electrostatic Charge Control
3.9.3.13.1 LEO Charging Design Standard
The spacecraft shall meet the intent of the requirements contained in NASA-STD-4005, Low
Earth Orbit Spacecraft Charging Design Standard. [R.CTS.285]
How the contractors achieve that might be proprietary. It looks like (at one time) SpaceX used electrically conductive paint[3] as part of their mitigations.
You missed this one: "International Docking System Standard
(IDSS)" [0] which I actually read that before answering hence my "required by the docking standard".
You will see that it says: "IDSS compliant mechanisms protect against electrostatic discharge through the soft capture system" which is to say not much.
That’s still more than the previous standard for which the paragraph on ESD just says "RESERVED".
TL;DR they dissipate charge using three Plasma Contactor Units (PCUs) on the Z1 truss, and measure it using the Floating Potential Measurement Unit (FPMU) on the S1 truss. This clamps the ISS to within 20 volts of the local plasma.
https://s3vi.ndc.nasa.gov/ssri-kb/static/resources/NASA-HDBK...
Apparently you attach all grounds to some common component, like the chassis and then you have a zero volt reference. As long as it is stable and the relative voltages to it are consistent, that should suffice for electrical systems. Handling overall charge of the craft might be a bigger challenge, but no idea how that would be handled.