Afaik, the New-VIProperty you defined is called for each cluster individually.
So there is no need to use the Group-Object.
Can you perhaps try it like this
#MemoryMinUsageMonth -MB
New-VIProperty-NameMemoryMinUsageMonth-ObjectTypeCluster -Value{
param($cluster)
Get-Stat-Entity$cluster-Statmem.usage.average-Start(Get-Date).AddMonths(-1) |
Measure-Object-PropertyValue-Minimum |
Select -ExpandProperty Minimum
}