Including multiple authors in Authoring information for a Drupal 7 node
I accomplished this by the following steps:


Download and enable the Rules module and the Entity Reference module.
Add a field called "Foo" to your content type. Field type: Entity Reference. Target type: user. 
Create a new Rule.

Add event: "Before saving content"
Add condition: Data comparison. Parameter: Data to compare: [node:type], Data value: bar (Where "bar" = your content type's name)
Add action: Add an item to a list. List: [node:field-foo], Item to add: [site:current-user]



Now every time content is saved, the user who saved it will be added to the list of editors of that page.