When you get an email with an invitation and accept it, the event or to-do is added to your calendar. If it is an event, a little icon is placed in the event in the Agenda view.
If you have an invitation in your KMail inbox you can either press to ignore the request, or press or . The last two actions will insert the item into your calendar. In any case, KMail will send your response to the organizer automatically.
You can also or the invitation. When delegating, you can suggest another person as your proxy. Using you can forward the invitation to one or more people not yet invited. When the receiver of the delegation or forward accepts the invitation, the organizer will be asked to approve the new attendee.
If for any reason you changed your mind, edit your status in the item's attendee tab. If you checked the Use Groupware Communication box in the KOrganizer group scheduling configuration, KOrganizer will ask you to send an email updating your attendee status. If not, choose the → menu item to send your status update.
If you are not satisfied with some of the characteristics of the event or to-do you got invited to, and want to send a counter proposal to the organizer (for instance an alternative location or time), just edit the item and send it back as your proposal, by selecting the item and choosing the → menu item.
The organizer of the event will get your proposal by email, and will be able to accept it or decline.