As an FYI to everyone using this plugin: If you want to give all logged in members access to this feature DO NOT use the everyone group, use trust_level_0. It was non obvious to me until I read that the everyone group actually includes even non-logged in users and that it’s generally not appropriate to attach permissions to this. (In practice, associating the calendar permissions with everyone does nothing at all - new users will be unable to access the feature.)
The plugin might do a better job of 1) eliminating everyone from the selection, 2) having some note to the admin to use trust_level_0 if that is their intention.