Workflow java API guide, page 25:
“A Workflow user group has a shared Inbox, the group sees all of the tasks
assigned to their associated roles. As such when one user removes a task from queue the entire group will receive notification, updating their Inboxes as well.”
This doesn’t make any sense. First they say the group has a shared (single) Inbox, then they say when one user takes a task from the queue, it updates all their (multiple) inboxes. What is the actual behavior of the system?
Such strange terminology for Workflow 
In Workflow there is no concept of groups, so this terminology is not quite acurate. I believe what the author of the documentation is trying to communicate is:
Workflow distributes tasks to users based on what roles they have been granted. All users with the same roles see the same set of Workflow tasks and all are eligable to accept and complete these tasks. When one user accepts a task, the entry is removed from the inboxes of all other users who have the same roles.
Basically a set of users share a view of a queue maintained on the Workflow Server. These queues are dynamically constructed on the server, where there is one queue for each role defined in the system. Workflow clients, such as a custom application that implements the Java Client API connect to the appropriate number of queues, based the definition of a user account. Multiple users therefore get to see the same set of work items, called tasks. However only one user is allowed to execute the task at any one time. When the client API is used, the server will send a notification to all client sessions that are connected to the same queue so that the inbox can be reconciled with the fact that the task has been accepted and is no longer available.
Excellent thank you. That clarifies things greatly. It occured to me the distinction might be between queue vs. inbox.