Hi there,
If you want to impersonate a user and that user exists in AD, then you need to create a login for that user before it can be impersonated. Unfortunately (perhaps) SQL Server cannot read individuals directly out of a group created as a login in SQL Server.
If you want an AD user to impersonate another AD user then you need to contact your AD team and see what they can do for you.
Regards,
Kev