There had been some requests to share a card without adding the requester as a member.
There is a power up "external share" that can do that.
I had used quite successfully with Contalist to share a card-like email with labels, checklists, attachments etc.
Here is a prototype to share a card. The objective is to add a level of authentication and a list of eligible cards for a user.
Let me know what other ways you have to share a card privately.
A short video is available here : https://youtu.be/HMXZ-4jAJhE