Script to add an entry to an access control list. Wrote this four years ago, so definitely not my best work.

Add-ACEtoACL.ps1

    <#
        .SYNOPSIS
        Creates an access control entry (ACE) and adds it to an access control list (ACL).

        .DESCRIPTION
        Creates an access control entry (ACE) and adds it to an access control list (ACL).

        .PARAMETER Rights
        A right or list of rights to be allowed or denied by the ACE.
        
        .PARAMETER Inherit
        Force the ACE to be inherited by child objects.

        .PARAMETER AccessType
        Used to specify whether or not the ACE allows or denies the specified rights. Accepts values of "Allow" or "Deny"

        .PARAMETER Identity
        The identity (Active Directory user account, group, computer account, etc.) the ACE applies to.

        .PARAMETER Object
        The path to the object whose ACL you are modifying.
        
        .EXAMPLE
        Add-ACEtoACL -Rights "Modify" -Inherit -AccessType Allow -Identity "contoso\cbabbage" -Object "C:\Data"
        
        This example allows the user cbabbage in the contoso domain to modify the C:\Data folder and all of its children.
    #>

  Param(
    [Parameter(Mandatory=$true)]
    [String]
    $Rights
,
    [Switch]
    $Inherit
,
    [ValidateSet("Allow","Deny")]
    [String]
    $AccessType
,
    [Parameter(Mandatory=$true)]
    [String]
    $Identity
,
    [Parameter(Mandatory=$true)]
    [String]
    $Object
)

$colRights = [System.Security.AccessControl.FileSystemRights]$Rights

if ($Inherit) {
    $InheritanceFlag = [System.Security.AccessControl.InheritanceFlags]"ContainerInherit, ObjectInherit"
}
else {
    $InheritanceFlag = [System.Security.AccessControl.InheritanceFlags]::None
}

$PropagationFlag = [System.Security.AccessControl.PropagationFlags]::None

$objType =[System.Security.AccessControl.AccessControlType]::$AccessType

$objUser = New-Object System.Security.Principal.NTAccount($Identity)

$objACE = New-Object System.Security.AccessControl.FileSystemAccessRule `
    ($objUser, $colRights, $InheritanceFlag, $PropagationFlag, $objType)

$objACL = Get-ACL $Object
$objACL.AddAccessRule($objACE)

Set-ACL -Path $Object -AclObject $objACL