This simple little usefull script can be used to create a portgroup at the cluster level. This enables you to create it once and have it be consistently deployed to all the ESX and/or ESXi hosts in your cluster.
foreach ($ESXhost in get-cluster CLUSTER_NAME| get-vmhost)
get-virtualswitch -vmhost $ESXhost -name VSWITCH| new-virtualportgroup -name PORTGROUP_NAME -vlanid VLANID
Simply replace CLUSTER_NAME with the name of your cluster, VSWITCH with the name of the vSwitch you would like to create the portgroup on, PORTGROUP_NAME wiht the name of your new portgroup, and VLANID on the VLAN you would like to associate with the portgroup. If you do not wish to use a VLAN remove the -vlanid VLANID portion of the script.