The final solution:

    @api.multi
    def mdo_indienen(self):
	self.ensure_one()
	#Attach all people that are in the group 
	user_ids = self.pool.get('res.users').search(self._cr, self._uid, [])
	user_ids_to_attach_as_chat_followers = []
	for user_id in user_ids:
	    flag = self.pool.get('res.users').has_group(self._cr, user_id, 'aa_sel_kempen.group_sel_manager')
	    if flag is True:
		user_record = self.pool.get('res.users').browse(self._cr, self._uid, user_id)
		user_ids_to_attach_as_chat_followers.append(user_record.partner_id.id)
	#Write on the record
	self.write({
	    'state': 'aanvraag',
	    'message_follower_ids': user_ids_to_attach_as_chat_followers
	})
	"""
	    Write a notification message on the chatter of this MDO.
	    This message should also be shown in the messages inbox for all admins.
	"""
	body = _(u'Er is een nieuwe MDO aanvraag ingediend voor de patiënt ' + self.mdo_patient_id.name + '.')
        return self.message_post(body=body, partner_ids = user_ids_to_attach_as_chat_followers