Hello Vish,
Users that access the namespace can share assets only with other users or groups in the same namespace. They can’t see users and groups in other namespaces.
Fore more info - Please see Namespace operations - Amazon QuickSight
To better understand Namespaces, Groups etc please see Understanding Namespaces, Groups, Users and Shared Folder in Amazon QuickSight
Hope this gives some insights.
Cheers,
Deep