@Override
  public boolean addMember(Member targetMember) {
    if (super.addMember(targetMember)) {
      MemberRollup memberRollup = (MemberRollup) targetMember;
      for (Map.Entry<String, Bucket> entry : getColocatedBuckets().entrySet()) {
        String region = entry.getKey();
        Bucket bucket = entry.getValue();
        Member member = memberRollup.getColocatedMembers().get(region);
        if (member != null) {
          bucket.addMember(member);
        }
      }
      return true;
    }
    return false;
  }
Represents the sum of all of colocated buckets with a given bucket id.
geode-core/src/main/java/org/apache/geode/internal/cache/partitioned/rebalance/model/BucketRollup.java