They distribute wristbands to guests arriving from the Epcot Resort Area through the International Gateway from 7 - 10 PM. They have a pretty efficient system set up. They had two people scanning your room keys with hand held devices (presumably to make sure you're legit), then you proceed to one of four castmembers applying the wristbands. I assume they scale back their staffing after the big rush is though.
I can't speak for what they do at the main gate, but I'd guess it would be the same.