Moving from Dropbox to SharePoint - structure and external sharing questions.

Dropbox is only a cloud storage service. Office 365 is much, much more. Hence when migrating to Office 365 you can choose among many variants.

As I told before, generally speaking, you could use several Groups (each Group comes with its own site collection and its own doclib). But you could very well also use subsites or even different doclibs in the same site collection.

I personally would probably go with Groups, but it's a delicate choice and I cannot give you a more precise advice without a deeper analysis of your use case.

About the sync problem, I would not wish to need Groove to my worst enemy! It's really a horrible piece of software: if you can, stay away from it!

Is it really *necessary* for your external users to sync to their destops? Or maybe it is only a "bad" habit?