Notifications are definitely tied to user accounts. This is how notifications are suppose to work.
1. If you don't login...or are not a subscriber...click a topic, read it and return to the Home page...the topic remains bold. It remains bold because your actions are not associated to a user account.
Then you log in...
2. If you click a topic and just read it...meaning, you don't contribute...the font thins. However, if another contributor changes the post...meaning, contributes to it...the font returns to bold because the post has changed and you haven't read the update.
If you contribute to the post, the font does the exact same thing: bold, thin, bold, etc with each new post. It's no different from you not contributing to the post.
3. Again, if you contribute to a post, the website automatically subscribes you to the post. If you didn't contribute, you're not subscribed but the font still indicates changes to the post. The red circles are subscription updates.
If you contributed to the post and then someone else contributes, you'll get a red circle(s). If you don't contribute, you'll never get red circle(s).
Bold Font indicates changes to any post and Circles indicate changes to subscribed post.
-----
I don't understand your "joining" question.