A common feature request that we receive from our users is the ability to define groups of users. Users want the ability to combine their folks into teams and then assign the team to a particular task in the workflow. One nice advantage this has is that it allows you to add and remove users from a task just by adding or removing them from a group. no need to edit the workflow in the designer just to drop a person from a task. This level of abstraction can help when building your workflows, by allowing you to focus more on the task and less on the who.
With the release of this group functionality we are also adding the ability to reassign a task responsibility to another person or group just for a particular workflow. We have long had the ability to reassign a person's responsibility if they are on vacation or left the organization but now we have the ability to do it for a single workflow.