In version 4.0 and up, you may now add byline/co-authors to posts. In order to use this feature, you must enable bylines in the settings. To use the feature on custom posts and pages, select them in the dropdown box before saving.
When bylines are enabled for a post type, the Byline Authors metabox should appear when adding/editing a post. Here, you can add and remove both Guest Authors and Users as byline/co-authors. These authors are in addition to the main author.
Decide how you want to display your co-authors based on the options available in the settings.