Norm,
An interesting question. What is the shutdown group for? I ran the following script on my AIX box running 5200-05 and found no users belong to this group, although it did exist...
for i in $(cat /etc/passwd | cut -d: -f1)^Jdo^Jlsuser $i^Jdone | grep shutdown
I then ran I find to see if I could find any files in this group...
find / -group shutdown
Both turned up nothing, I must assume the answer to your question is no need to create a shutdown group, nobody belongs to it, and it nothing is set to it.