Co-authors defined by authors possible ?

Hello,
Thanks for that usefull plugin.
I just wanted to ask if it would be possible to give the ability to authors to define their own co-authors. It seems (but I’m not sure of that) that only an admin could do this for the moment.
If this is not possible, an alternative for me would be to define authors groups that could edit the same posts. Is that possible ?
A last alternative : give restricted permission to an editor to be able to edit only the posts written by a certain author. I would say that here I’m going out of the scope of the plugin isn’t it ?
I’m not sure I was clear :/ Sorry for that if ever…

Thanks in advance for the reply. 🙂

http://wordpress.org/plugins/co-authors-plus/