Is it posible to send Tempo messages from within a process instance?...

May I ask about the business case?
Are you truly trying to post a message (as a specific users viewable to that person's followers)? Or will making a post as a user or system to a specific group meet the case? The system event and post as users are possible via smart services.

The use case is as follows:

Every process instance has a finite number of responsible users

Every process instance generates a news post which is a space for the responsible users to discuss, this discussion must be private and only accessible to the responsible users

Every time one responsible user posts a comment, all the other responsible users must be notified

Every responsible user can invite one or more users (outside the responsible users group) to the discussion. Once invited, the new user's comments are notified to the whole group and the whole group's comments are notified to them.