“who would be interested in an accountability group where everyone writes and publishes an atomic essay every day for 30 days”

Gate cohort entry with one habit requirement

Don't onboard people into a complex curriculum on day one. Strip the entry requirement to a single observable daily action and let community accountability carry the rest. This is what scaled from a single tweet into 500-1,000 paying students per cohort and ~10K total — one habit, one promise, one measurable streak.