/_api/web/getFileByServerRelativeURL('/docs/Test.docx')/ListItemAllFields/SharedWithUsersId

{
  "odata.metadata": "http://.../_api/$metadata#Collection(Edm.Int31)",
  "value": [
    6,
    15
  ]
}

{
  "odata.metadata": "http://.../_api/$metadata#Collection(Edm.Int31)",
  "value": [
    6,
    15,
    25    <---- User just added
  ]
}

{
  "odata.metadata": "http://.../_api/$metadata#Collection(Edm.Int31)",
  "value": [
    6,
    15,
    25    <---- User has been removed from share list
  ]
}