As teamwork becomes ever-more important in a new age of remote work, it is critical to develop metrics to quantitatively evaluate how effective teams are. This is especially true with mixed-modality teams, such as those that include a human and an agent or human and robot. We propose a novel utilitarian metric, the Harmony Index, which quantifies the effectiveness of team members by classifying them into four sub-types based on the result of their teaming on overall effectiveness. This index is evaluated using a real-world dataset of over 1 million interactions, and potential future uses of this index are explored in the realm of team science.