If you’re talking about MK Disney After Hours, for recent events (summer+) they had a station set up in Tomorrowland outside of the former Stich ride, by the FP kiosks. There was another one (I think but not totally sure) next to Philharmagic. The Liberty Square ticket office is another option as PP suggests. For early 2018 MK DAH events, many did have to leave/re-enter (it was a common gripe on the DAH thread, but hopefully Disney continues with the inside the park stations).
If you’re asking about the DHS or AK Disney After Hours events, we don’t know yet exactly how those will be handled.