You can obtain what you want ordering $UserInfo
array by the number of properties, it can be done, but it's not so simple, in your case I would add another propertie with the count of groups added:
...
ForEach ( $GroupName in $GroupMembership ) {
$objUserInfo | Add-Member -MemberType NoteProperty -Name CtxGroup$counter -Value $GroupName.SamAccountName
$counter++
}
$objUserInfo | Add-Member -MemberType NoteProperty -Name NbCtxGroup -Value ($counter - 1)
$UserInfo += $objUserInfo
...
And then order descending the array on this propertie :
$UserInfo | Sort-Object -Property NbCtxGroup -Descending | Export-Csv .\F.csv
It's not so nice but it will do the trick.
Have a look at :
$objUserInfo.psobject.Properties | Measure-Object