Hey there @East64582 - great question!

At this time there is no perfect way to set up an event type that takes into consideration the availability of a Calendly user that is not part of your organization (or someone that is not a Calendly user at all). 

Right now, Calendly will only consider the hosts assigned to an event type within your organization. The best way to make this work would be to add the additional host to your Calendly organization. If they do not wish to be added using their current personal Calendly account, you can send an invite to a secondary email address for them and they can join you, creating another Calendly account that they use for your joint meetings and endeavors. They would be able to connect the same calendars to this second Calendly account that they have connected to their current account, ensuring that double bookings/conflicts aren’t created between accounts (example: if they have meetings booked via their personal Calendly account and connect the same calendar to their account with you, their second account will read those conflicts and show them as unavailable at those times). 

Otherwise, the only way to make this work would be to add them to your calendar events after bookings have been made. Example: if an invitee books a time with you and that booking is added to your Google Calendar, you can then add the additional host to the Google Calendar event. This “busy” event will be read by their Calendly account (if they have their calendar connected) and they will then be marked as unavailable in their own Calendly account.

I will loop this feedback to our product team, as well! =)