Designing effective haptic feedback is challenging due to the subjective nature of touch and the fact that multiple people cannot easily share and evaluate touch experiences. In this work, we propose CoTacs, a collaborative haptic toolkit to address the challenges of haptic feedback design by enabling designers to explore haptic experiences together using group collaboration and allowing them to refine ideas quickly through early feedback during the design process. Two design sessions with five collaborators each show that CoTacs enables users to design, evaluate, refine and explore haptic feedback together. Participants leveraged group feedback to improve their designs and inspire creative ideas for the second collaboration round. Our work demonstrates how collaborative haptic toolkits can enable synergistic effects through interactions between the designers, benefiting haptic feedback design while still exposing several limitations. We discuss different opportunities for the design of future collaborative haptic toolkits and haptic group collaboration techniques.