One way I can think of is to use the Sheets widget in the courses they take to record their last sessions. And, in a separate Storyline project, build a logic to filter all Sheets data by date or data range. Having this list, you can use the Email widget to send emails to those people. That's one way. But really, this is something you should discuss with your JavaScript developer. We provide the technology, how you use it - it's up to you really.