Hello @Mark_Raouf welcome to the Asana Community Forum 
There has been a similar discussion yesterday, have a look here Sharing customized Asana Project Externally - #2 by Andrea_Mayer
Multihoming tasks can work here (Hosting the task in various projects).
But keep in mind that if a task is added to your “shared project” also means the same comments, notes, assignees show there.
So depending on what you need you might want to create separate tasks for this purpose or subtasks which are added to the other project.
But yeah you have to be careful and ensure your team then does not post anything there that should not be seen by the external party.
And here is a post, related to how much access to give clients, that you might be interested in: 
Dealing with external clients - How much access should they have?