Getting Office 365 groups in sharepoint CSOM with powershell - powershell

Question

I've created an security group through Sharepoint Administrator portal, let's name it "group1". and I have a website, with a list and in that list there is a person/group column field, let's name it "PersonGroupColumn".
When I add a new item to my list, i can find "group1" when I type into "PersongroupColumn". Although, what I want to do is to set up a powershell script that updates all the items of my list with "group1" in "PersonGroupColumn"
Here my starting script:
# Get Start Time
$startDTM = (Get-Date)
# Global variables
$User = ""
$Pwd = ""
$SiteURL = ""
# Functions
function addUser
{
param([string]$DisplayName,[string]$Group)
$d=New-Object PSObject
$d | Add-Member -Name DisplayName -MemberType NoteProperty -Value $DisplayName
$d | Add-Member -Name Group -MemberType NoteProperty -Value $Group
return $d
}
#Libraries
[System.Reflection.Assembly]::LoadWithPartialName('Microsoft.SharePoint.Client') > $null
[System.Reflection.Assembly]::LoadWithPartialName('Microsoft.SharePoint') > $null
[System.Reflection.Assembly]::LoadWithPartialName('MsOnline') > $null
# Password Init
$Password = ConvertTo-SecureString $Pwd -AsPlainText -Force
# Connection to Office 365
$Creds = New-Object System.Management.Automation.PSCredential($User, $Password)
connect-msolservice -credential $Creds
# Gathering groups and members
$array = #()
$Groups = Get-MsolGroup
foreach($Group in $Groups)
{
$Members = Get-MsolGroupMember -GroupObjectId $Group.ObjectId
$line = New-Object psobject
foreach($Member in $Members)
{
$array += AddUser $Member.DisplayName $Group.DisplayName
}
}
# Connection to sharepoint Online
$Context = New-Object Microsoft.SharePoint.Client.ClientContext($SiteURL)
$Creds = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($User, $Password)
$Context.Credentials = $Creds
# gathering the list
$Web = $Context.Web
$List = $Web.Lists.GetByTitle('Rapport de visite')
$Context.Load($Web)
$Context.Load($List)
$Context.ExecuteQuery()
$Query = [Microsoft.SharePoint.Client.CamlQuery]::CreateAllItemsQuery()
$Items = $List.GetItems($Query)
$Context.Load($Items)
foreach($Item in $Items)
{
$AffectedGroup = ""
$Item['Groupes'] = ""
foreach($Member in $array)
{
if($Item['Author'].LookupValue -eq $Member.DisplayName) {
$AffectedGroup = $Member.Group
}
}
if($Item["Groupes"] -ne $AffectedGroup) {
$Item['Groupes'] = $AffectedGroup + ";"
$Item.Update()
$Context.ExecuteQuery()
}
}
# Get End Time
$endDTM = (Get-Date)
# Echo Time elapsed
"Elapsed Time: $(($endDTM-$startDTM).totalseconds) seconds"
It seems like I need to create a fieldUserValue object and push it into the item's field, but I need the LookupId of the group and I don't know where to get it.
Some help would be really appreciated.
Thanks!