custom user levels

Greetings,

WordPress is almost the perfect media for my current project, with the exception of one thing:

I need to have a user who is capable only of editing a post to which he was assigned ownership by admin. Said user cannot post new articles.

I have searched plugins and the forums for other information about custom user levels, and so far I’ve come up empty-handed.

I found some discussion, but alas my php is quite rudimentary and the development topics are beyond my current capacity to comprehend. I have looked at capabilities.php, but I can’t find where the roles are really defined.

It seems that I am limited to the 5 Roles defined in the code. I know that there is a variety of ‘capabilities’ that are assigned to the different roles, but those roles appear to be statically defined. Are there any plugins or features that allow a more granular control over user levels?

Thank you,

Darryl G.