How to synchronize data between different users?

Question

I’m new to programming and I’m trying to create an app. I’m trying to figure out how I can synchronize the data between a group of users. For example, I want an application where one user can add another to a "group" and then they can manage the same information. If someone changes a field in the app, the information would have to be updated on the other devices in the group as well.

Note: Each one of them having its own user.