The fifth generation wireless system (5G) is expected to handle with an unpredictable number of heterogeneous connected devices while guaranteeing a high level of security. This paper advances a groupbased Authentication and Key Agreement (AKA) protocol that contributes to reduce latency and bandwidth consumption, and scales up to a very large number of devices. A central feature of the proposed protocol is that it provides a way to dynamically customize the trade-off between security and efficiency. The protocol is lightweight as it resorts on symmetric key encryption only, hence it supports low-end devices and can be already adopted in current standards with little effort. Using ProVerif, we prove that the protocol meets mutual authentication, key confidentiality, and device privacy also in presence of corrupted devices, a threat model not being addressed in the state-of-the-art group-based AKA proposals. We evaluate the protocol performances in terms of latency and bandwidth consumption, and obtain promising results.