As a result of the rapid developments in digital technologies, smart cities have been adorned with several digital gadgets based on the Internet of Things (IoT). The essential part of smart city operations is IoT communications. IoT devices generate, process, and exchange a large amount of security-critical and privacy-sensitive data regularly, resulting in attractive targets for threats and attacks. Group-oriented communication, such as data gathering and area monitoring, is critical in the IoT World. It enables users to manage various IoT devices simultaneously. Conventional one-to-one authentication techniques donot consider the resource constraints of IoT devices in grouped communication. They also do not solve the issue of massive machinecommunication (mMTC) scalability. Many to Many (M2M)authentication approach of group authentication makes grouporientedcommunication and mMTC very secure. The lower timecomplexity of Group-based authentication (GBA) makes theseprotocols very popular for efficient and secured communication.This paper uses a polynomial-based group authentication schemeand membership verification to ensure efficient and threat-freecommunication among IoT devices. Bi-variate polynomials havebeen used instead of single variable functions in the proposedscheme. The protected nature of the bi-variate polynomial makesthe proposed scheme very secure and reliable. Furthermore,establishing the session key makes the proposed scheme effectiveand efficient. Security analysis of the proposed work shows itsefficiency over existing schemes.