@Sally_H As I understand, https://activitypub.ghost.org/ is the blog/newsletter about building Ghost’s ActivityPub features, and the “get early access” probably only applied to Ghost(Pro).
If you’re self-hosting, from what I understand, you need to either use the Docker Compose install that is in preview, and host the ActivityPub stuff yourself, or else proxy ActivityPub traffic to ap.ghost.org using Nginx/Caddy/etc.
But I’m no expert because I haven’t gotten it working either, even though I have the redirect to ap.ghost.org set-up :)