$wUrl  = "http://contoso/sites/ITS"
$w = Get-SPWeb $wUrl
$l = $w.Lists["Accomplishments"]
$gr = $w.Groups["Accomplishments Viewers"]
$readRole = $w.RoleDefinitions["Read"]
$ra = new-object -TypeName "Microsoft.SharePoint.SPRoleAssignment" -ArgumentLis $gr
$ra.RoleDefinitionBindings.Add($readRole)

Write-Host -NoNewLine "Start "
$l.Items | % {
    $_.RoleAssignments.Add($ra)
    $_.SystemUpdate($false)
    Write-Host -NoNewLine "."
}

Write-Host " done"