What permissions does a role need for the user to be assigned as the author of a post?

You probably need to add 3 capabilities: edit_published_posts publish_posts delete_published_posts but at the very least, you need to add publish_posts as that is the capability WordPress uses to determine what users are shown in Authors drop-down. Note: Changing capabilities is stored in the database so the recommendation is to modify these values via a plugin … Read more

grant multiple roles access to specific admin menu item

You shouldn’t use Roles to handle permissions for your menu page. Instead, use capabilities, and then assign those capabilities to whichever roles should have access. For example, instead of creating a customrole role, use a custom capability like manage_custom_plugin: add_menu_page( ‘Custom-Plugin’, ‘Custom-Plugin’, ‘manage_custom_plugin’, ‘custom-plugin’, ‘init_custom_menu_page’ ); Now, using the Members plugin you can enter manage_custom_plugin … Read more