Different permission for different user on Excel Online

Try this: This uses Jquery and a small plugin to generate add to Calendar and If you wish even using ical/vcal to users desktop calendars Works like a charm https://github.com/tardate/jquery.addtocalendar

With Dropbox, to keep things simple, "You can't share a folder that contains a shared folder."

So no. If you have a folder that is shared, all of its contents are then shared.

But yes, with having different access level data in different folders, this would be possible. As you describe, have a shared folder "Main", and another folder "TOP_SECRET".

It's also worth noting, that if you wouldn't care about actual access rights, but only the convenience of what each party has on their computer, this would be possible. You can change include/exclude the folders that are synced to a specific computer by configuring the client on that computer.