which attribute can you configure on the route reflector to allow routes to be identified as belonging to the same group?

Together, a route reflector and its clients form a cluster. When a single route reflector is deployed
in a cluster, the cluster is identified by the router ID of the route reflector.
The bgp cluster-id command is used to assign a cluster ID to a route reflector when the cluster has
one or more route reflectors. Multiple route reflectors are deployed in a cluster to increase
redundancy and avoid a single point of failure. When multiple route reflectors are configured in a
cluster, the same cluster ID is assigned to all route reflectors. This allows all route reflectors in the
cluster to recognize updates from peers in the same cluster and reduces the number of updates
that need to be stored in BGP routing tables.
Reference. http://ieoc.com/forums/t/5326.aspx