Hi Yogesh - do those external portal users have Active Directory accounts that are licensed users on your Tableau Server? If so, yes.
External users would authenticate to the external portal with their AD identity using whatever mechanism your external portal uses; the external portal would relay their identity via trusted authentication to the Tableau Server, so Tableau Server would know which user they are and show them the appropriate content. How Trusted Authentication Works explains in more detail.
Internal users would log on directly to Tableau Server with their AD identity.
Hope that helps!
I done it using trusted authentication.