curl --request GET \
  --url https://api.shiftkeeper.io/schedules/{schedule_id}/slack_user_group/ \
  --header 'x-api-key: <api-key>'

{
  "user_group": {
    "id": "S083J1KTL9X",
    "name": "Incident responders",
    "handle": "incident-responders"
  }
}