Android : How do I share data between several android units

heres what i want to do, I want to be able to share data from a sqlite database between users on different android devices. I have some rows in my table that needs to be synced across multiple devices. I guess i should rely on an external database to synchronize the data but what remote database structure is the premier and how do i send the data?, as JSON or what?

Also how do i notify the other device application that theres is updated information to get from the server?, or however i choose to do it.