As @Emily_Roman clarified here above, the portion of @Bastien_Siebman’s comment about people needing to be Asana users is no longer true; as long as someone has an email address with a domain that matches the domain of the Asana organization, they can participate in request tracking.
It is still true that external users whose email address domain doesn’t match the organization’s domain can’t participate in Request Tracking. And a view-only license wouldn’t help such people because such a license would require them to have a domain email address.