SPUtility.SendEmail

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<string> emails =newList<string>();
   
 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);
 
   }
}