The Internet of Things (IoT) enables software to orchestrate physical spaces. This results in high security requirements for IoT communication in factories, households, or critical infrastructures. Besides point-to-point communication, group communication is frequently used in the IoT. It has to be secured, which typically requires the exchange of cryptographic keys. Several protocols have been proposed for Group Key Management (GKM). They vary in their targeted settings, in their Key Distribution Model, Architecture Model, Reliability Properties, and Protocol Overhead. This paper surveys existing GKM mechanisms, analyzes their suitability for constraint IoT settings, and identifies open issues that require further research.