There exists a wordpress setup, having a closed user group (ie one cannot produce a login for themself). Whenever I write your blog &lifier surveys are published into it, I recieve up-to-date over mail. But, how you can make certain to update other customers also.
I'd consider using a Wordpress subscription wordpress plugin. The renowned wordpress plugin with this is known as Subscribe2. It works best for registered customers. Otherwise, browse Wordpress's Extend page for the same one. I guess fixing your condition such as this does not allow it to be a programming problem, why re-write code?