A new great functionallity would be invitations, so you can invite other users to your appointment just like outlook has.

Michiel Klaver

I think that existing client side code allows to add extra buttons or sections to send invitation, and multi-select support, which was added in 2.3, allows to link multiple persons to the same event.

Full server side integration will require a lot of not-flexible code - email processing, additional tables in DB , etc.