Members of a primary-backup group in a clustered computer system are organized into subgroups to manage primary and backup resources being managed by the group. Group members are placed into subgroups based upon their access to particular resources, such that a primary subgroup may be defined comprised of members having access to a common primary resource, with one or more backup subgroups defined comprised of members having access to a common backup resource. A join protocol is used to determine to which of a plurality of resources managed by the primary-backup group a joining member has access, and to add the joining member to a subgroup for a resource to which the joining member has access.