E-ride nights start at whatever is the normal closing time for Magic Kingdom. Tonight, MK closed at 7:00 pm, so the e-night started at 7:00 pm and runs until 10:00 pm. On nights that the MK doesn't close until 8:00 pm, e-night would be 8-11 pm. And so on and so on.
Yes, once the park is closed, the only persons allowed on the rides are those with the plastic bracelets who purchased e-ride night tickets.
Tickets are $12.83 , including tax. You need to purchase them at your resort Guest Services area. You have to prove that you are a WDW resort guest and how many people are registered in your room.