Originally, the PublishPress Authors was only available as an add-on for PublishPress. The name was “Multiple Authors”.
However, the add-on proved really popular and people constantly asked us to release it as a standalone plugin.
Multiple Authors version 1 and 2 required the PublishPress plugin to be installed.
Multiple Authors 3 is an independent plugin with its own menu links in the WordPress admin area:
If you already use Multiple Authors, you should be see this update inside WordPress, as in the image below:
We've spent a lot of time making sure that we maintain backwards compatibility. However, as with any site update, it's always worth taking a site backup first. In particular, be careful if you'd made any significant changes to author layouts in your theme or Multiple Authors itself. We're here to help if you need us.