In my view both are fine, depending on context.
If the volunteers are expected to be already sitting in their seats (for example, you're standing outside the auditorium just a few minutes before the event starts), then you would say "by now''. But if you were looking some time ahead -- say to the next day -- then you would say "by then".
In another interpretation, "taken" could mean "booked", in which case different example scenarios apply but the principle is the same.