I'm running a game for a discord server and by its nature people will drop in and out. So I wrote it into the overarching plot. Reality is literally coming apart at the seams. Objects and creatures from other worlds have been leaking in. At the same time things have been dissappearing without a trace. Not just stuff either, an entire city just vanished in a flash of green light. A powerful wizard is investigating and needs adventurers to go out and do the grunt work. They've been traipsing about the planes trying to find the missing city and some clue as to what is happening.
There's also a monk who's working for the wizard. His job is to escort adventurers where they are needed via portals, meaning people will occasionally appear mid-adventure. Sometimes people slip out of reality. They experience it as a few seconds but the real time elapsed varies.