The module will be available for Odoo 9.0 soon. You have time to buy Internal Messaging before price increase! (When you purchase 8.0 version, you get a right to download 9.0 and any further module versions).
By default, to send a private message to specific recipient(s) you need to delete other followers included to the same document. The module allows to send private messages to recipients you specified, regardless of who are in the followers list.