#Connect-MgGraph -Scopes "Group.ReadWrite.All", "User.Read.All"

#Target Group ID
$TargetGroupID = "GroupID"
$TargetGroup = Get-MgGroup -GroupId $TargetGroupID


#Extract groups from .yaml into a readable array
$groups = Get-Content -Path "filepath.yaml" -Raw | ConvertFrom-Yaml


#variable nulls
$Yaml_Members_Source = $null
$AAD_Members_Target = $null
$fullGroup = $null
$Yaml_Ids = $null
$AAD_Ids = $null
$Full_Details_Add = $null
$Full_Details_Remove = $null


#hashsets creation
$Yaml_Members_Source = New-Object System.Collections.Generic.HashSet[string]
$AAD_Members_Target =  New-Object System.Collections.Generic.HashSet[string]


#source hashset population
foreach ($group in $groups) {
    $fullGroup = Get-MgGroup -Filter "displayName eq '$group'"
    [String[]]$Yaml_Ids = @(Get-MgGroupMember -GroupId $fullGroup.Id).Id
    $Yaml_Members_Source.UnionWith($Yaml_Ids)
}


#target hashset population
[String[]]$AAD_Ids = @(Get-MgGroupMember -GroupId $TargetGroupID).Id
$AAD_Members_Target.UnionWith($AAD_Ids)


#Determine users to add and remove from Target
$Users_To_Add = $Yaml_Members_Source.Where({$_ -notin $AAD_Members_Target})
$Users_To_Remove = $AAD_Members_Target.Where({$_ -notin $Yaml_Members_Source})


#Add users missing from Source to Target
ForEach ($ADD_member in $Users_To_Add) {

    try {
        $Full_Details_Add = Get-MgUser -UserID $ADD_member -ErrorAction:SilentlyContinue
        }
        catch {
            Write-error "User ""$($Full_Details_Add.UserPrincipalName)"" was not found"
        }

    try {
        New-MgGroupMember -GroupId $TargetGroupID -DirectoryObjectId $ADD_member -ErrorAction:SilentlyContinue
        Write-Information "User ""$($Full_Details_Add.UserPrincipalName)"" added to ""$($TargetGroup.DisplayName)"""
        }
        catch {
        Write-Debug "User ""$($Full_Details_Add.UserPrincipalName)"" is already a member of ""$($TargetGroup.DisplayName)"""
        }
}


#Remove users that are in Target but not in the Source
ForEach ($Remove_member in $Users_To_Remove) {

    try {
        $Full_Details_Remove = Get-MgUser -UserID $Remove_member -ErrorAction:SilentlyContinue
        }
        catch {
            Write-error "User ""$($Full_Details_Remove.UserPrincipalName)"" was not found"
        }

    Remove-MgGroupMemberByRef -GroupId $TargetGroupID -DirectoryObjectId $Remove_member
        Write-Information "User ""$($Full_Details_Remove.UserPrincipalName)"" removed from ""$($TargetGroup.DisplayName)"""
}