Attend/Not Attending the Event

I Created : Post Type: Contact Taxonomy1: Groups Taxonomy2: Events Metaboxes : - Name - Position - Address after adding my contact list, and selecting which Contact are invited to Event how can i add checkbox (or dropdown).. with this option: Attend/Not Attending i need a funciton/Plugin allow me in Admin Area to add who attend the Event, buy Clicking on a checkbox or anything else please note that there are lot of event, and each contact may be invited to many event so the relation should be between Post type - Taxonomies - Checkbox. Thanks