You could create a group "all" which is private - hidden - no tags.
Every user is member of this group. So you cloud mention the group @all where all users are members.
FYI: I use a plugin so I will get notification when someone registers
I'm not sure you will want to get an email any anyone makes a post.
But you can watch a thread and get an email notification when someone posts there.
That's built in check your user profile settings.. Again, even this can become too much as your forum grows.