I am using a BigQuery service account to impersonate a user without any Oauth process. The user with my domain gets impersonate but how can I impersonate the user outside of the domain. I have given the user outside my domain a service account user role in service account grant permission section and also added the service account in the IAM permissions of the user account with BigQuery editor and BigQuery job user role. After all these steps when I try to access the user's BigQuery data I get "Client is unauthorized to retrieve access tokens using this method, or client not authorized for any of the scopes requested."
What can I do to impersonate a user outside my domain.