Allow specific user edit rights

Can anyone suggest a way of allowing specific users restricted access to edit specific posts? More ideally, this would be access to a group of users but a single user would be a good start.

I don’t think that the author of a post can be changed in Admin but it is not a problem to do this in phpmyadmin.

Just to clarify, I want to add posts as admin and then provide access so that a specified user(s) can modify them (but nothing else).