Collaborative Computer-Aided Design (CAD) plays an important role in engineering design by enabling simultaneous multi-user work on design tasks. However, the absence of a unified framework to describe collaborative CAD and associated design activities has resulted in inconsistencies in research findings and challenges in synthesising existing knowledge. This paper aims to develop a taxonomy that describes the usage of collaborative CAD and identifies research opportunities based on the current state of the art in collaborative CAD and related design activities. Using Activity Theory, six top-level taxonomy categories are proposed to structure collaborative CAD activities: Subject, Object, Tools, Community, Division of Labor and Rules. Following the PRISMA methodology, a systematic literature review of collaborative CAD activities was conducted and combined with the grounded theory to derive the third- and second-level taxonomy elements and subcategories. Additionally, five research opportunities that highlight emerging challenges, including the integration of AI tools, the need for interdisciplinary collaboration, and the development of standardised evaluation metrics are identified. The findings have practical implications for researchers, software developers, and educators by providing guidance on future research directions and collaborative CAD tool development. By offering a comprehensive taxonomy based on both the theory and existing literature, and research opportunities, this study contributes to the ongoing evolution of collaborative CAD in engineering design, paving the way for more structured research in this field.