Vehicular social networking (VSN), as a novel communication paradigm, exploits opportunistic encounters among vehicles for mobile social networking, collaborative content dissemination, and to provide a variety of services for users and their vehicles. VSNs promise to solve problems such as the ever-increasing number of road accidents, or traffic congestion, by forming networks of vehicles whose users share common interests. However, in particular in urban regions, VSNs benefit from extending far beyond vehicular networks with their road side units, by integrating all kinds of sensors to provide higher accuracy and additional information in increasingly crowded areas. Integrating such sensors turns VSNs into networks of cellular-connected IoT devices with mobile cells and raises the question how to authenticate all these units in a scalable, efficient, and anonymous manner. In this article, we present SI-AKAV, an efficient group-local authentication scheme for cellular-connected IoT devices, which is specifically tailored to overcome the high computational costs of existing schemes, while improving in all of the above dimensions. For example, compared with the state-of-the-art authentication and key agreement protocol (AKA), SI-AKAV reduces communication volume by 29% and computation overhead by 41%.@en