First, in a feature receiver, I swapped the Event content type with the Schedule content type to take advantage of the Attendees field (and the cool Free/Busy field!):

SPList list = lists["Calendar"]; SPContentType newContentType =     list.ContentTypes.Add(list.ParentWeb.ContentTypes[SPBuiltInContentTypeId.Schedule]); SPContentType oldContentType = list.ContentTypes[0]; string name = oldContentType.Name; string description = oldContentType.Description; oldContentType.Delete(); newContentType.Name= name; newContentType.Description= description; newContentType.Update();

The form now looks like this:

Second, I added an Event Receiver that sends the notices:

publicoverridevoidItemAdded(SPItemEventProperties properties) {
        SPList list = properties.List;     SPListItem item = properties.ListItem;     SPFieldUserValueCollection values = item[SPBuiltInFieldId.ParticipantsPicker]asSPFieldUserValueCollection;     List
    
      emails =newList
     
      ();     foreach(SPFieldUserValue value in values)     {
            SPUser user = value.User;         if(!string.IsNullOrEmpty(user.Email))         {
                emails.Add(user.Email);         }     }     if(emails.Count>0)     {
            StringDictionary headers =newStringDictionary();          headers.Add("to",string.Join(";", emails.ToArray()));         headers.Add("subject", item.Title);         SPUtility.SendEmail(web, headers, body);      } }