How to group arrays in Powershell?

Adding to @mklement0's great answer using Group-Object, we can also group the items into a System.Collections.Hashtable by adding the -AsHashTable switch. To iterate the hashtable keys and values, we have to use System.Collections.Hashtable.GetEnumerator.

$objects = @'
Number,Color
1,Red
2,Red
3,Red
4,Red
5,Blue
6,Blue
7,Green
8,Green
9,Green
'@ | ConvertFrom-Csv

$ht = $objects | Group-Object -Property Color -AsHashTable

$ht.GetEnumerator() | 
    Select-Object @{Name="Color";Expression={$_.Key}}, @{Name="Number";Expression={$_.Value | Select-Object -ExpandProperty Number}}

Output:

Color Number
----- ------
Green {7, 8, 9}
Red   {1, 2, 3, 4}
Blue  {5, 6}

If we want to sort your result by Number, we can use Sort-Object to sort by the first number:

$ht.GetEnumerator() | 
    Select-Object @{Name="Color";Expression={$_.Key}}, @{Name="Number";Expression={$_.Value | Select-Object -ExpandProperty Number}} |
        Sort-Object @{Expression={$_.Number[0]}}

Or use an System.Collections.Specialized.OrderedDictionary to create an ordered hashtable of arrays and maintain order of key insertion:

$ht = [ordered]@{}
foreach ($object in $objects) {
    if (-not ($ht.Keys -contains $object.Color)) {
        $ht[$object.Color] = @()
    }
    $ht[$object.Color] += $object.Number
}

$ht.GetEnumerator() | 
    Select-Object @{Name="Color";Expression={$_.Key}}, @{Name="Number";Expression={$_.Value}} 

Which will output the following:

Color Number
----- ------
Red   {1, 2, 3, 4}
Blue  {5, 6}
Green {7, 8, 9}