Hey ​@L.L , to my knowledge this is not possible at the article or topic level, rather the component level. I look forward to learning otherwise if someone else replies

Examples:

You could create a private or unlisted Group and then invite the people you want to be able to access the content to the group. 

You could create a forum (Community > categories) that is only visible to a Custom Role you create.

This is really the only way out I've found, but it doesn't work very well for me because there will be new people not yet registered in the community! Thanks for your help

You can control the visibility to a custom role and effectively make it so only the person in that custom role can see it.  You can also do a custom page with similar permissions.

If it’s for people not yet registered, I have a few articles that I controlled the visibility to only unregistered role, so only guests can see it.   How to register and the like.