I think spark:all is not sufficient for GET /people.
Per the docs: https://developer.webex.com/docs/api/v1/people/list-people
Searching and viewing People requires an auth token with a scope of spark:people_read [must specify Email, displayName, role, or id list]
Viewing the list of all People in your Organization requires an administrator auth token with spark-admin:people_read scope.
