tl;dr: Never run events alone—always have a co-organizer.
One of the easiest (and most fun!) ways to make sure you’re replaceable is to never run a meetup alone. If you organize a meetup, find a co-organizer and split the responsibilities with them. This helps avoid burnout, plus it’s a great way to strengthen your friendship.
My friend Will and I organized the Austin Effective Altruism meetup together for a year. When one of us got busy with life, the other was able to keep things going. And when I moved to Berkeley, the meetup just automatically kept going—I didn’t even need to appoint a successor, there already was one.
I follow a similar model running Authentic Relating Games at REACH. I try to always have a co-facilitator—it’s way more fun, takes some of the pressure off, and it’s an easy way to train potential replacements for myself.