To associate the a post with a user you would need to have either a user field type on the post of a post object field on the user, or both and code them to be bidirectional.
However you set this up you’d need to have an acf/save_post filter that creates the relationship when a new user is added. Basically, something along the lines of when a new user is added a new post is automatically created for that user and this post is automatically added to the post object field for the user.
