Permissions in Multiple Authors
This guide has useful information about controlling the permissions in the Multiple Authors plugin.
By default, Multiple Authors follows all the standard rules for WordPress permissions. If someone has permission to edit a post before Multiple Authors is installed, they will still have the permissions after installation.
Permissions and Guest Authors
If a WordPress user is selected to be one of the authors of a post, they will have the permissions to edit that post.
There is one exception to this: Authors who are in the “Contributor” role can only edit posts if they have not been published. This follows the normal core behavior in WordPress.
Permissions and Orphan Posts
Multiple Authors does allow you to have posts with no authors.
If a post has no author, editing permission is controlled by the “ppma edit orphan post” permission. You can control who has this permission by using the Capability Manager Enhanced plugin.