I spent several hours yesterday searching for a plugin that could. I didn’t find anything that was built to do this. I think what I’ll end up trying is a http://www.doodle.com/ scheduling poll shown in an iFrame, and then creating the Event separately. It’s not ideal to use 2 admin systems but it should be simple to implement. I’d definitely pay for an Events Calendar add-on that brought doodle style group scheduling poll capabilities into the WordPress admin.