When creating departments or any other object that accepts options per user, they should support the use of objects based on groups. - For Example... 100 users to be added manually is an effort wrong ..
This is by design. The developers intentionally limited this to user objects because of the performance impact that the resolution of group members has on the overall reporting. If you really feel that this is a needed feature, please head over to our feature portal and file a feature request for this.