Pretty straight forward example and reference for adding rights to a folder or file with PowerShell.
Example
$AccessControl = get-acl "D:\My Documents"
$AccessRule = New-Object Security.AccessControl.FileSystemAccessRule("Users", "FullControl", "ContainerInherit, ObjectInherit", "None", "Allow")
$AccessControl.AddAccessRule($AccessRule) set-acl -aclobject
$AccessControl -path "D:\My Documents"