As organizational structures shift from centralized and hierarchical structures to decentralized and loosely coupled networks, value creation and innovation processes are changing as well. Especially in the services sector, we observe a shift from closed and company-internal innovation processes to more open and collaborative forms of innovation (e.g. via various types of communities that emerge in enterprises' surrounding ecosystems). Communities of co-innovation or innovation networks can be described from a service systems perspective. In this work, a modelling approach to generally describe service systems is developed leveraging the St. Gallen Media Reference Model (MRM). A specific instantiation of this model is presented that allows for modelling service systems in terms of communities of co-innovation. The model is applied for the case of co-innovation in the enterprise software ecosystem of the SAP AG. The approach fosters understanding, organizing and managing such new networks of collaborative innovation in service science.