Groups allow you to enter an overall target for several individual items; the target applies to all items within the group. Each individual item within the group can still have its own target.
You can add a new group to a question, by right-clicking the question and selecting add group. Similarly, you can create a new group containing an existing response by right-clicking the response and selecting add group.
You can add other items to a group simply by dragging and dropping theme there.