I have two node Veritas Cluster and I have requested 3 more LUN in existing Cluster..
All 3 are attached, but 1 is already assigned to a group (not sure how it is created )
Please show us what you see?
Post output of 'vxdisk -o alldgs list' on all cluster nodes, highlighting the lun that you are referring to.
Also let us know SF/HA version and OS.