Concrete5 - Automatically email notification when removed from gruop

Hi I've running a membership website via Concrete5 CMS via the standard group setup. I use the automatically remove user functionality, where the user is removed from a specific group at a certain date. What I need is to be able to send out an email telling the user, that there is X days until the membership ends. E.g. 14 days, 7 days and 1 day before removal. The code should work on a standard Concrete5 installation and use cron job for checking. It should be pretty simple, as Concrete5 handles automatically notifications a lot. Thanks