For messages: You can set the default notification level for groups to watching. But you can’t prevent people from changing that later after they join the group.
For topics it is already set to normal by default, which means that when you @ mention groups they will be notified.
But maybe you are talking about category settings. You can set this for new users in /admin/site_settings/category/user_preferences.
There is a plugin for enforcing this for users later to prevent them from unwatching a category.