You're inviting people in batches and access depends on order — a beta, a limited launch, an invite wave.
A spreadsheet or a newsletter list has no position and no ordering guarantee. "Who's next" becomes a manual sort, and a double-submitted form silently creates two rows for the same person.
Position is assigned server-side inside a locked transaction and returned in the same response as the signup. Re-submitting the same email is a no-op — the original position comes back, not a duplicate.
A real API key, the MCP server, and signup emails — no credit card.