カテゴリに新しいトピックが追加されたときにユーザーにメールで通知することはできますか?

Have them change Category to watching

The SS above shows it from topic controls. They can add a category to watched in Preferences.

Alternatively There is a theme-component that can make this easier as well.

There is also a plugin that allows you to follow members.

「いいね!」 2