You may subscribe any partner to any object. Just push the button 'invite' in the top left corner. Be cautious: when sending a notification to your partner Odoo generates a signup url for the portal.

Besides, this module may be helpful to your purposes: https://apps.odoo.com/apps/modules/9.0/compoze_no_auto_subscribe/

Hello,

First of all I am very disappointed by looking on the unrelated answers over here.

Here I put my thoughts with hope that you are a Odoo technical programer.

Actully functionally this is not possible unless there is some custom module developed for this feature. Only fields which are related to "res.users" class, they can be added to auto subscribe by configuration in Odoo.

*Important Tip* : Any field which is related to "res.users" class they can be easily added into auto subscribe by inherit this method :  _message_get_auto_subscribe_fields ( Your class must extend mail.thread ) 

However Partner field in Helpdesk form is related to "res.partner" class, so you have to develop custom code with override the write method like as follows,

In your custom module extend the crm.helpdesk object and add following method,

    def write(self, cr, uid, ids, values, context=None):
        """ Override to add case management: open/close dates """
        if values.get('partner_id') :
            self.message_subscribe(cr, uid, ids, [values['partner_id']], None, context=context)
        return super(crm_helpdesk, self).write(cr, uid, ids, values, context=context)

You have to override create method as well. So at the creation time, Selected Partner will automatically add in the followers area.

By adding above write method in your custom program, when you edit Partner in Helpdesk screen, the new partner will automatically added in followers area.

This is code which is tested and from one of my custom modules. So I am sure it will work.

Good Luck ! Let me know the result !

Hope this helps,