Centerness is defined as a quality of multi-agent systems (groups) where agents share a common set of system goals and interact so the system will achieve those goals. A pair of measures is identified to capture the two dimensions of centerness: distance-weighted fragmentation and average goal centerness. As a case study, the measures of centerness are applied to six teacher professional development groups within the Information Technology in Science Center for Teaching and Learning. The calculated measures of centerness of these groups generally conform to the expectations. Insights on using this measure of centerness to evaluate centerness in other professional development programs are included.