We're shutting down HubSpot, so we should send all future mailing list signups to the blog's member list.

There is apparently a /members/ endpoint
https://forum.ghost.org/t/adding-and-removing-members-api/10353/3?u=auryn
You can see the code for the API here: https://github.com/TryGhost/Ghost/blob/3.0.2/core/server/web/api/canary/admin/routes.js#L87-L88
Looks like router.post('/members/csv'... might be what we want
DM me for API keys.