Add someone to existing 2 people Messenger chat

Macbook Air & iPhone 12 Pro

I have been chatting to someone for some time and it is a long conversation. We want to have a friend join in our existing chat. How can we add them so they can see everything we have said and be able to chat with us?

If this is not possible, how can I copy and paste the whole conversation? I don't want screenshots because it is all in a different language and the person will have to copy and paste into a translator.

I am desperate to do this! It is about our family genealogy.  

Thank you