'When active_model is res.partner, the current partners should be attendees'

@api.model
def _default_partners(self):

    partners = self.env.user.partner_id
    active_id = self._context.get('active_id')
    if ((self._context.get('active_model') == 'res.partner') and active_id):
        if (active_id not in partners.ids):
            partners |= self.env['res.partner'].browse(active_id)
    return partners
