I made few sales group for example North, South, East, West and configured such that users from each region can only see their "OWN DOCUMENTS ONLY" and removed access from "Sales : All Documents" and "Sales : Administrator". 

Now from what I believed the function of Add followers was that a user can access any cases from any sales group if they are invited to be the followers. I discussed about this to the Odoo Team to which they replied that Add followers is only applicable for users who can access all documents. 

Now for example how do I ensure that a user from North if given permission can collaborate with South on a single case only if he adds that user as a follower and not give access to all the cases of North with the South User.

Please do help since I believe this should have been a basic functionality if they gave an option to create Sales Groups.