What is the best way to sync data among a small number of users in swift?

As the header indicates, I am looking for the simplest way to sync user-generated data (Integers, Booleans, NSDates, etc) among a small number of individuals (at this point, I am just thinking of sharing data between two people). Within the app, users can populate an array with instances of a custom object and this data is used to populate a UITableview. Assuming all users in the select group have synced their devices they should all see the same data in the tableview.

My original idea was to write to a json file in a shared Dropbox or Google Drive folder. After looking around online, however, I found that this method is likely to lead to data corruption. Cloudkit only allows public or private (single account) syncing, nothing in between. I have seen some posts that recommend using Parse, but that service is now on its way out.

Does anyone know of a (preferably free) way to do this?