Getting all users using the SDK is currently not supported. All other available options were already posted by Vachas.
What’s wrong with using the RestConnector as Vachas has suggested?
https://www.cumulocity.com/api/core/#operation/getUserCollectionResource
Please follow the community rules and don’t open 2 threads to the same topic, also please read the answers carefully.
If you think that is something that should be available in the SDK you can create a feature request here: https://cumulocityiot.ideas.aha.io/