These are the ways I see collaboration can be achieved with Tableau Server:
1. Users can post comments about different worksheets and read what others users posted.
2. Users can filter to the data they want and then click "Share" for a direct link they can send to others.
3. Users can filter to the data they want and then select "Remember my changes"
Tableau could also serve as the interface for visualizing data within a collaborative environment. Tableau Server visualizations could be embedded in a web forum or in SharePoint. That would give a rich interactive environment in which people could see the data and collaborate.
One thing we've done for a client is to wrap Tableau Server in a web interface which allows users to leave feedback and also captures run-time application errors. We've got a couple of Tableau dashboards that give us good insight into the feedback and errors. We can see errors by customer, description, frequency, and time. This allows us to more effectively and efficiently meet the needs of our customers and provide a better experience. This could be expanded into a much fuller and richer collaborative experience.