GetNestedMembership

Function Get-NestedMembership {

    [CmdletBinding()]
    param (
        [Parameter(Mandatory = $true, ValueFromPipeline = $true, Position = 0)]
        [object[]] $InputObject
    )

    Begin {
        $users = New-Object System.Collections.Generic.List[object]

        #region INNER FUNCTION
        Function getRecursiveMembers {
            param (
                [System.Collections.Generic.HashSet[string]]$Processed,
                [string]$DistinguishedName
            )

            $users = New-Object 'System.Collections.Generic.List[object]'

            $ldapFilter = "(memberOf=$DistinguishedName)"
            [array] $objs = Get-ADObject -LDAPFilter $ldapFilter

            foreach ($member in $objs) {
                if ($member.ObjectClass -eq "group" -and $Processed.Add($member.DistinguishedName)) {

                    $innerResult = getRecursiveMembers -Processed $Processed -DistinguishedName $member.DistinguishedName
                    $Processed.UnionWith($innerResult.Processed)
                    $users.AddRange($innerResult.Users)

                }
                else {
                    $member.psobject.Properties.Add((New-Object PSNoteProperty("FromGroup", $DistinguishedName)))
                    $users.Add($member)
                }
            }

            [pscustomobject]@{
                Users     = $users
                Processed = $Processed
            }
        }
        #endregion
    }

    Process {
        foreach ($group in $InputObject) {

            $processed = New-Object System.Collections.Generic.HashSet[string] -ArgumentList 1
            [void] $processed.Add($group.DistinguishedName)

            $result = getRecursiveMembers -Processed $processed -DistinguishedName $group.DistinguishedName

            foreach ($u in $result.Users) {
                $u.psobject.Properties.Add((New-Object PSNoteProperty("MemberOf", $group.Name)))
            }

            $users.AddRange($result.Users)
        }
    }
    End {
        $users
    }
}