Access to everything in Carnival, including adding new users to the app.
Author access allows the user to do just about everything in Carnival, except add new users to the platform, delete the app, generate API keys, etc. As an author, you may notice that most of the options in Settings disappear.
You will be able to send messages.
Developers can update key app settings screens, but will not be able to publish messages from the Messages screen. They can, however, send test messages to devices in Developer mode as long as you also have a Developer push certificate uploaded for iOS devices. This doc walks through that process a bit, but you'll see the "Send Test" option in Settings / Logs here.
Just make sure the devices is first marked as Developer, or you won't see this test option.
The user will only see the audience they have permission to see, as well as messages published to that audience. If you want to give a user access to multiple audiences, you'll need to create the multiple audiences under a Parent audience, and then give the user access to that Parent audience.