Are you specifically concerned about getting the same group together or only continuing to run a campaign you care for?
The most important factor in a campaign starting and continuing is having a group of players who are invested in it and who want to make it work.
When I got back into TTRPGs with the release of 5e I know from the start that it would be difficult to juggle family and work obligations with my desire for an ongoing campaign and I wanted to organize it in a way that would allow for flexibility so others in a similar situation could similarly have a group to participate in.
The main guidelines for organizing my campaign from the beginning continue to serve our group well, please a more recent change that allows us to continue. There are:
1. Carefully select the group
My first two players were old high-school friends that I got back in touch with when I moved back near where we grew up. The others I found by posting to a local Meetup Group. Before posting I spend a lot of time thinking about the kind of game I wanted to run and wrote up a pitch that was part movie-trailer type advertisement, to give a flavor of the setting and campaign, and a detailed list of expectations, approach to rules, how scheduling will work, etc. There was also quite a bit of back and forth with potential players by e-mail before the first session. Since then there has been some turn over but all new players have been referrals from existing players.
2. Flexible Scheduling
For many groups, it seems that having a set schedule with set expectations for attendance is the way they can ensure lasting groups. But I knew that would not work for me. Between work demands and travel and my kids' school and extracurricular activities, it would not be possible for me to commit to a set schedule.
At the end of each session we pull out our calendars and plan the date of the next session.
3. Longer but fewer session
For many people, it is easier for them to work in shorter sessions. Many groups keep to 4 hours for a weekly or bi-weekly session. Some go as short as 2 hours but maybe every week. I have a friend that runs hour-long sessions at work during lunch break.
That doesn't work for me. Because of the nature of my work, it was easier to find a day than a few hours, due to my work requiring travel. Also, I find short session unsatisfying.
I run on eight-hour session each month on a Saturday.
3. Conclude adventures or milestones each session to allow flexibility for no-shows and drop-ins
Generally, I work hard to avoid ending a session with the party in location or situation where we'll have to pick up and continue at the point the next session. That way if someone can't make it the next session we don't have to have someone else play their character or retcon anything. It also makes it easier to work in guests how join for a single setting.
In my first campaign, which was a home brew setting with a mix of adventures I created myself and materials I got from other sources, it worked out well. We eventually settled on milestone leveling where every 1 or 2 sessions that party would level up and the adventure for that day was designed for a party of that level.
My second campaign, was Curse of Strahd. It was a bit harder but for the most part I managed to end each session on a downtime location.
My current campaign is a mega dungeon (Rappan Athuk). The party has a stronghold nearby and we generally end each session with them returning to their stronghold.
All this helps avoid anyone feeling like they are "missing out" when they can't make it or resentment of those who have to miss a session.
Because of this, I've had people who have had to drop out and quite a while later are easily able to come back into the game when their real-life situation changes.
4. On-line Sessions
Recently I took a job that requires long periods of time overseas. I continued to run the games mostly using the tools I've always used, but with the additional of using Google Meet for voice and screen sharing (to show maps and images, etc.)
I'm finally getting serious about looking into using VTTs. But you don't need VTTs if the cost or learning curve is intimidating. I'm still not sure if and when I'll actually switch to using VTTs because the technical hassels and extra prep time keeps putting me off of using them. The main reason I'm considering them is that I miss the tactical play of my in-person games and it is getting difficult to have satisfying combats in interesting locations at higher levels without at least dropping miniatures/tokens on a map.
Tools to consider during social-distancing include:
For campaign management: World Anvil (there are others, I just like this one). That way you have a way for folks to keep track of the campaign, even if you have to go long periods between sessions. It also gives a nice way to keep active in the campaign between sessions.
For teleconferencing: Discord (voice, video, and chat) seems to be the big winner among gamers. I personally hate it but I'm in the minority it seems. My favorite is Google Meet/Hangouts. Hangouts is the free version. I would recommend going with Hangouts not just because it is free but has dice-rolling functionality built into the chat. Skype and Microsoft Teams are also good. Most VTTs have integrated audio/video but experiences with the quality and reliability vary. Google Meet/Hangouts has never let me down.
VTTs: for ease of starting, go with Roll20. Neither you nor your players have to install any software and you don't need to mess around with firewalls, port forwarding, etc. If you want all the bells and whistles, have a LOT of maps, large maps, and want to try to combine campaign management and virtual battlemaps in one product, go with Fantasy Grounds. I've been playing with the beta of Fantasy Grounds Unity and I like it, except networking continues to be a pain in the rear. If you like to homebrew and customize and want a developer and community that is supportive of that, give d20pro a whirl. If you are short on money but are comfortable with technology and are patient, check out Map Tools.
Scheduling: to make scheduling easier, check out Doodle.
Hope it helps and good luck! With the current pandemic scare I think us gamers are well-prepared to weather the social distancing than many. With all the tools available now we can continue to engage in an activity full of creativity, camaraderie, cooperation, and significant periods of time engaging with others beyond quick calls to "check in." Take advantage of it!
