The Hadoop Group can be changed with the following command:
/var/lib/ambari-server/resources/scripts/configs.sh \
-u admin -p admin set localhost cluster_name cluster-env user_group new_group
This assumes it is running from the Ambari host with the default credentials. Replace cluster_name with the name of the cluster.
Details here: https://cwiki.apache.org/confluence/display/AMBARI/Update+Service-Accounts+After+Install