Every mahjong group starts the exact same way: four people, and at least three of them have no idea what they're doing yet. That's completely fine. You're not casting a tournament team. You need people who'll actually show up.
Start smaller than feels ambitious. Two people who want to learn, plus you, is already a real start. The fourth person usually turns up faster than you'd expect once there's an actual group to invite them into: a neighbor, someone from work who's mentioned being bored on weeknights, a post in the local Nextdoor or Facebook group. "Looking for a fourth" is one of the more common, and more successful, asks in those spaces right now.
Pick the night before you pick anything else. A group that meets "whenever everyone's free" never meets. Ever. Same Thursday, every other week, beats a theoretically-better night that only actually happens once a month.
The first few sessions will be slow, and that's fine too. Someone's always learning the Charleston in real time while the table waits. It gets faster than you'd think, usually within three or four sessions. Resist the urge to simplify the rules to speed things up. The game clicks once people have genuinely played it enough to stop thinking about it, not before.
Once everyone's got a few hands under their belt, the table itself starts mattering more than you'd guess going in. A mat people actually like, snacks that don't leave a mess on the tiles, lighting that doesn't make anyone squint. Small stuff, but it's usually the difference between a group that fizzles after two meetups and one that's still going strong a year in.
Honestly, finding four decent people was never the hard part. Picking a night and refusing to let it slide: that's the whole game.
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