Can I limit the permission to view posts/topics only to the user creator?

So I want to know if it’s possible to hide posts based on authorized users, such as user creator and the user that created that post/topic and add other users, so they can also see it?

Also, unrelated, but I couldn’t find if it’s possible to add comments/replies via API

Sounds more like using group DMs. Users can add other users to group DMs.

Admins can control various permissions to categories based on groups, but regular users cannot set individual access to posts, topics or categories - that is what DMs are for.

API documentation:

Official Discourse API documentation: