Giving permission to edit specific posts

Hi! I have two roles: Administrator and Editor.

I would like the Editor to be able to create posts (including posts from custom post types), but not to publish them or edit other’s posts.

I have successfully done that, but I would also like the Editor to be able to edit specific posts created by different users but ONLY if an Administrator grants them permission to do so/”adds them” to a specific post.

Would that be possible to set up? If so, how?

The page I need help with: [log in to see the link]