I don’t know a lot with a FTP above 300w, and most don’t ride at FTP every ride. So I will guess the average Zwifter generate below 200w for a hour.
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I was taking 50 Watts into the grid as a starting point. Assuming that your average cyclist might generate 200 Watts at the pedal. The lost 150 Watts would represent losses due to losses in the generator, powering fans etc and transfer to the grid.
Our indoor cyclist would indeed feel resistance as the generator is driven. Assuming generator efficiency is around 95%, our cyclist would experience 200W of resistance and would generate 190W of electricity.
190W minus your fan, laptop and losses due to transfer to the grid… you may have a little left. I need to find out how much power my fan uses!
Exert 200W on pedals.
Output with 90% efficiency drivetrain/generator - 180W
Minus fan (mine is 30W apparently) - 150W remaining
Minus laptop, turbo power supply etc - 50W remaining
I don’t run the fan at max power and the laptop/turbo power supplies probably do not draw the maximum value stated on them constantly. There would be losses in transfer to the grid no doubt but I expect you’d transfer 50W.
9 cyclists per house.
Do you also have lights on where you Zwift?
I really don’t think you would be at a positive when all is said and done.
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I don’t generally have lights on but the main light in the room is a 5W LED I believe. This probably wouldn’t impact the calculation much.
It’s an interesting idea. Obviously if you ended up with positive value, it would be small but perhaps in the future the infrastructure will be in place, so any energy produced would feed back regardless of how small. It would be satisfying if we could harness the power of the 10,000+ people cycling right now.
I run 2 x 60w fans and one 130w fan. Currently all on mid level. In the summer I run 4 fans on full. Just to get rid of all the heat I generate. The human body is not very efficient in generating power. Most of our energy is turned into heat.
That sounds intense. I should imagine you’re putting down more than 200W too 
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it’s all fun and games until you decide to zwift during a power outage and give the workers fixing the power grid an extremely mild shock 
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‘extremely mild’ 
Another way of looking at it is; feed the 200W back to the grid, and that’s effectively 200W less you’re consuming. Making us more environmentally friendly.
It may be an idea which is more common in the UK. We have smartmeters which record what you feed back into the grid. Normally from PV panels, but also from domestic wind and water. I should imagine an indoor turbo is similar to a small domestic wind turbine.