Hello Paul. #user# is not an available variable when setting up a Domain Event. You could set up an alias containing all users of the domain, and then have this Domain Event send there, which would then forward to all domain users.
Also, in the Domain Events condition, I would use 'Greater than 90%' instead of '=90%'. If your domain had a sudden influx of messages and your total domain size went from 89% to 91%, this Domain Event would not be triggered.
Hope this helps.