PowerShell to Update Multi-valued Peole Picker Field with New Users

Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue

#Configuration parameters
$SiteURL = "http://portal.crescent.com/projects/"
$ListName = "Projects"                
$FieldName= "ProjectTeamMembers"
$UserToAdd="Crescent\Faris"
$ItemID=431

#Get site and List objects
$web = Get-SPWeb $SiteURL
$List = $web.Lists.TryGetList($ListName)

if($List -ne $null)
    {           
        #Get the Item
        $Item = $List.GetItembyID($ItemID)

        #Get Existing field value
        $MultiUserCollection = [Microsoft.SharePoint.SPFieldUserValueCollection]$item[$FieldName]

        #Prepre the user to add
        $User = $Web.EnsureUser($UserToAdd)
        $NewUser = new-object Microsoft.SharePoint.SPFieldUserValue($Web, $User.ID, $User.LoginName)

        #Update Multivalued people Picker field 
        $MultiUserCollection.Add($NewUser)
        $item[$FieldName] = $MultiUserCollection
        $item.update()       
    }
write-host "New User Added and Existing People Picker field value updated!"