You don't need a plugin to set up authorship for multiple authors.
All you need is that all your authors have a Google + account.
If you go into Users in wordpress and set up your users/authors. ( I presume you already have these users set up as authors if they are already writing content?). When they are set up as an author, you go into the user profile and add their Google+ profile link into the section on their profile where is says Google + Profile URL. You do this for each author on the site.
Then, when any author writes a post, they just need to ensure they are writing as their user profile name (i.e. they are logged in as themselves) and that should cover it.
The admin can change the author of a post by doing a quick edit on the list of posts - when it drops down there should be an 'author' section where you can select a list of authors. This list is your users and each of these need to have their Google+ profile url added to their user profile in wordpress.
Hope that makes sense!