What to do with UUID of group of files?

In many cases, you don’t even need individual files UUIDs. You can address each file in a group by indexing the group:

http://www.ucarecdn.com/d51d18b3-5150-4153-b4c6-0a70f36663db~2/nth/0/
http://www.ucarecdn.com/d51d18b3-5150-4153-b4c6-0a70f36663db~2/nth/1/

If you need to get individual file UUIDs, it is better to subscribe to the onChange event using JavaScript API:

widget.onChange(group => {
  const files = group.files() // array of file instances
  Promise.all(files).then(infos => {
    console.log(infos) // array of fileInfo objects
  })
})