When someone joins my community and leaves
This is where commons fail. If they don't follow something.

Their not going to know about any new content

I need to keep in touch with them

The way I would do things is by using mentions.

I write a post that I want to engage people with

and I then mention people in the post

@Sally @Sam @Donna

The mention then email the users.
which in forms them about the post and to join the conversion