I run my game every Sunday 1 PM to about 5 PM. Unless I have some family thing, I’m sick or have some issue that would prevent me from showing up, I’m there. I don’t expect that my players will always make it. I encourage them to be there but I don’t expect it. My players are adults. They have families. They have jobs which sometimes interfere with scheduled game times. They have life. This is their hobby and social gathering that they do for enjoyment.

There are a lot of different approaches to dealing with this. You can have a game that is episodic. Each session is its own unique set of events that are only incidentally connected. The classic Greyhawk dungeon or West Marches campaign are examples. At the beginning of the session, you go exploring, at the end of the session you come back.

The way I do it is to make up reasons why the party member isn’t there and what they might be doing while everyone else is adventuring. I do that in a way that fits with what I know about the character, it might be humorous and I often involve the other players  by ruling that the party member is off doing what another player has suggested. Often it is something absurd or the character is doing guard duty, taking in the tourist sites or studying with a mentor. It doesn’t matter. What matters is that when the player for that character is present, they get to contribute in a meaningful way and when they are out, it isn’t something that impairs the character from quickly getting back into the game.

Because I’m not concerned with my story , whether there is continuity, a predetermined outcome or the sudden appearance or disappearance of a character not making sense as part of a narrative or a simulation; I just slot characters in as soon as they appear. No big deal. I am concerned with whether or not everyone at the table is enjoying the game. All other considerations are second to that basic principle. What I have learned is that no one seems to care if Bob the fighter is gone in the middle of the adventure because his player is on a family vacation this week. Does it affect the way they approach their adventure? Yes but they don’t care that the narrative was disrupted.