Assign post to a specific author

Hi,

there is probably a very simple answer to my question, but I have been searching for one for hours and searched the forum, and I can’t find it:

How do I assign a post to ONE specific author?

I have chosen the status “assigned”, but can’t find an option to choose the writer I would like to work on this post. He and myself should be the only ones allow to write on this post, whereas other can only see the progress (and maybe comment), but not write in it?

Thanks a lot for your help!

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