If your organization already has a directory, you can now make use of it from within the cloud using the AD Connector directory type. This is a gateway technology that serves as a cloud proxy to your existing directory, without the need for complex synchronization technology or federated sign-on. All communication between the AWS Cloud and your on-premises directory takes place over AWS Direct Connect or a secure VPN connection within an Amazon Virtual Private Cloud. This service also allows you to set up and run a new, Samba-based directory in the Cloud.
Read more here – https://aws.amazon.com/blogs/aws/new-aws-directory-service/