A Mobile Ad-hoc NETwork (MANET) is collection of mobile users that offer communication over a shared medium. Multicasting plays an important role in group-oriented communications. The combination of multicast services with ad-hoc environment introduces new challenges towards security issues. It is necessary to provide secure services like authentication, data integrity, access control, confidentiality etc for secure communication. Out of this, confidentiality is most important and it is achieved by key management. Clustering is an effective technique of key management in MANET. Cluster formation involves election of a node as cluster head and it controls other nodes in the newly formed cluster. The association and dissociation of nodes to and from clusters disturb the stability of the network topology.