I have the option of adding multiple authors to the same post and I'd like to be able to allow the guest author to be able to edit the post as well?
There isn't such kind of built-in feature within Toolset or WordPress.
In WordPress, one post can have only one post author.
There is a workaround, in your case, it needs to setup many-to-many relationship between posts and WP users, but Toolset relationships are based on post types.
So you can try to setup a intermediate post type "Members", use it to setup the many-to-many relationship between posts and "Members". For example:
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