Each of your external users will need their own Tableau Online or Tableau Server account, which would be used to authenticate/authorize them to view embedded Tableau dashboards. If you have a Tableau Server Core license, and turn on the Guest User feature, they could access the views as Guests.
Here is some documentation on embedding Tableau Server dashboards:
And on the Guest user:
The Guest user is only available with Tableau Server Core licenses, it is not available for Tableau Online or Tableau Server User licenses.