The quick and dirty version is that planning time for people of various ages and jobs is hard. So my goal is have each 3-4 hour session be an episode of a larger story. We have a fairly regular schedule (every Sunday that doesn't have our local soccer teams playing because most of us blog about those teams) but there are still vacations, weddings, movings, etc that get in the way. As soon as the one session is over I ask who can come. There are also two groups at this time, because a selection of people who hadn't played DnD ever decided to join several sessions in.
Those that can't are sent emails or digital chats about what their character is doing during their offscreen time. This Sunday I'm doing a double session so that I can get all 10 PCs on the same calendar. Group 2 is going 1st. Group 1 is 2nd. Here's a practical example.
Group 2 is currently nine days behind the rest of the campaign. These three characters are on a scouting mission and will discover a huge new hook separate from the others. That hook will almost certainly force them to flee, at some point. And they'll back to the current "base." This is only their second session and only 1 character has leveled up so far. The others are like 50 points out.
Group 1 is the other seven. Only three are making this session. They are chasing two major hooks and a side mission. I have all three sketched out enough that they will choose their own path and continue those storylines.
Samul, Saffron and Behn are the only ones that will be active on Sunday. Each has some clues that they've gotten through play or offscreen conversations.
Let's take the others into account.
Aamar is researching through a library and a Lorebook why magic is lessened and what's up with this group that kept the Kin (the peoples of this land) from having magic for thousands of years). This relates to one of the major hooks.
Maurice is trailing Parun, an ambassador for a slave nation who knows that the Group has a Lorebook and protected one of his escaped slaves.
Mansaray is the escaped slave. He separated from the Group when a crazy Scholar blew up a volcano. He doesn't know that Parun is chasing him, but he does know where the group is. When his player can return there are ways to get this Ranger back into the story.
Shonie started the early sessions, but due to the player moving out of town IRL is now a key NPC. If the Group has to flee the town they are in she will go with them and offer her services as help whereever they go.
One of my keys to managing this is to maintain the
story-blog. All the session recaps are there, so that what is public knowledge is there for everyone. Also, rules that go from test to active are there too.
Another key is the calendar. They don't get that, though one player keeps his own. But here I track key events. In this case Group 2 is about to discover that a volcano a few days travel away will erupt. How they act when that happens is up to them. These kinds of tricks from writing epic fiction (think Wheel of Time) help the world feel more alive.
Third, characters that are offscreen/table are discussed when appropriate. Parun mentioned Mansaray, even though that character wasn't in play for many sessions. During the last Group 1 session Aamar's studies were recapped to all present, so that they knew that the character was still active.
Lastly, keep the drop-ins involved via email, chat, twitter. Make certain they read the session recaps and ask them a couple questions about how their character behaves during their X days of downtime. Maurice also used a scroll that created a transmutation stone, similar to the Abjurer ability. Things like crafting, or social discovery can help them maintain a connection to the game, even while their real life interferes with the game sessions. When they come back they'll still believe their character is part of the group, even if the player hasn't been.
It's a lot of administrative stuff, but it adapts what happens in my favorite epic (length) fantasy fiction in that the party splits up and remixes regularly. They are all working towards shared goals and personal goals. Sometimes those overlap and others times they do not.
I've been struggling with how to write this up, but I think this made me think more about it, so thank you for asking.
