add subscriber following work item category/team area
One answer
Here is an example that does create an approval and also looks into categories: http://rsjazz.wordpress.com/2012/11/30/a-create-approval-work-item-save-participant/ this shows how to use the subsciptions api, however, honestly the API on the server should look very similar and fit right into the code above: http://rsjazz.wordpress.com/2012/08/02/subscribing-to-a-work-item-using-the-java-api/
Comments
I should have said, the code above finds the users by roles in the process area associated to the work item (which is determined by the category's association to the team area).
In addition I think that subscribing all the users of a team is a bad practice really and I would rather suggest the team to set up the team feed and have a good feed reader installed locally. That way the users will still be able to find some other mail in their in box.