#Set-PowerCLIConfiguration -InvalidCertificateAction Ignore
$vcenterServer = YOUR_VCENTER_SERVER_FQDN
$cluster = YOUR_VCENTER_CLUSTER
$esxiUser = "root"
$esxiPass = "YourMama"

$newUserName = "NameOfTheUserYouWantToAdd"
$newUserDomain = "DomainNameOfTheUserYouWantToAdd"

Try
{
    Connect-VIServer -Server $vcenterServer

    $esxlist =  Get-Cluster $cluster | Get-VMHost
    foreach($esx in $esxlist)
	{
        Connect-VIServer -Server $esx.Name -User $esxiUser -Password $esxiPass

        New-VIPermission `
        -Entity (Get-Folder -NoRecursion -Server $esx.Name) `
        -Principal (Get-VIAccount -Domain $newUserDomain -User $newUserName -Server $esx.Name) `
        -Propagate $true `
        -Role (Get-VIRole -Name ReadOnly -Server $esx.Name)

        Disconnect-VIServer -Confirm:$false -Server $esx.Name
    }

    Disconnect-VIServer -Confirm:$false *
}
Catch
{
    Disconnect-VIServer -Confirm:$false *
    Write-Host "Error occured while processing. Exit."
}

#Get-VIPermission -Principal (Get-VIAccount -Domain $newUserDomain -User $newUserName ) | select-object *
#Remove-VIPermission -Permission (Get-VIPermission -Principal (Get-VIAccount -Domain $newUserDomain -User $newUserName ))