The same question is relevant for situations where electric vehicles can’t be charged from outlets wired back to the meters of the individual units. Then, you would have outlets for vehicles out in the carpark wired back to the OC’s meter.
The solution is the same. A simple kWh meter can be put in line with each outlet (or one for the whole laundry). It can be read periodically and the relevant units billed pro-rata for the total cost to the OC, whether the outlet is in the allocated parking space of unit X or it has the washing machine belonging to unit X plugged into it.
There are fancier systems involving RFID cards for EV charging where different vehicles might use the same outlet and I expect the same could apply to washing machines. These systems record consumption to different users according to who tapped on and off.
Another option would be to make a reasonable estimate of the likely consumption of washing machines and dryers used for a certain number of hours a week (or measure the actual consumptions) and resolve to charge a fee to those using the laundry that would recoup that cost.