Co-Authors Plus is an old and popular WordPress plugin.
By default, WordPress allows you to choose a single author for content. Co-Authors Plus was a solution to this, because it allowed you to assign multiple authors to one content item.
However, Co-Authors Plus has only had a small number of updates in the last few years. Also, it can be hard to use because it requires you to edit your template files.
The Multiple Authors add-on for PublishPress is a fork of Co-Authors Plus that's worth checking out.
We reviewed the main PublishPress plugin, which is a really good editorial calendar. Multiple Authors is an extra feature for that main plugin.