K2 provides delegation features out of the box. Every manager in the directory has access to their subordinates’ tasks. As a user you can redirect your worklist items to anyone in the directory as well.
there is also a nice out of office component that allows users to setup out of office scenarios as well.
When you want a group of users to have exclusive access to business process tasks so that they can manage them, you either make this group of users as a destination in those tasks of interest and leverage the K2′s out of the box task management features OR will need to use K2 APIs to implement this functionality.