OpenERP Meeting Invitation . 

In Core OpenERP 6.1 meeting invitations are going wia mail and ICS file as attachment. But no feature for the Get back Response Or the Normal Invitation Feature which Mail Client are providing by default for accepting or rejecting invitations.

You can have this feature by providing . "METHOD : REQUEST" with valid Dates and organizer with "mailto" attributes.
"cal.add('METHOD').value = 'REQUEST' "

also you have to set the header for the mail .

"part = MIMEBase('text', "calendar") "
"part.set_param('method','REQUEST') "