We just attended an e-ride night for the first time in March. You purchase your ticket at your resort check-in desk. After 4:00 pm you can turn your ticket in for a wristband at one of the following places in the MK:
City Hall
Splashdown Photos
Tomorrowland Arcade
At park closing (our's was 7:00 pm), the rides/attractions that are not part of e-ride night will close their entry lines and shut down after those in line have ridden. At the entrances of the rides that ARE part of e-ride night, the CM will ask to see your wristband and will only allow those who have them into the line. According to my watch, those without wristbands were let in until about 7:05, but then were turned away after that time.
Unbelieveably, the park does clear out rather quickly. E-ride night is AWESOME and I would highly recommend it to anyone who has the opportunity to attend.