You can reply to an appointment, task, or note just as you can to a mail message. When you reply, you can include everyone who received the original item or you can include the sender only. Your reply includes the original message.
In the Calendar, tap the item you want to reply to.
Tap .
In the menu, tap Reply to reply only to the sender.
or
Tap Reply All to reply to the sender and all recipients.
Type your message, then tap Send.