I got this to work by adding this to functions.php…
add_filter('coauthors_plus_edit_authors', '__return_true');
…then using the User Role Editor plugin to add a capability named coauthors_plus_edit_authors and grant it users with the Author role.
I suppose the latter step could be done (programmatically?) without the User Role Editor plugin, but I’ve got it installed, so I used it.
