Since the very beginnings of cryptography many centuries ago, key management has been one of the main challenges in cryptographic research. In case of a group of players wanting to share a common key, many schemes exist in the literature, managing groups where all players are equal or proposing solutions where the group is structured as a hierarchy. This paper presents the first key management scheme suitable for a hierarchy where no central authority is needed and permitting to manage a graph representing the hierarchical group with possibly several roots. This is achieved by using a HMAC and a non-hierarchical group key agreement scheme in an intricate manner and introducing the notion of virtual node.