Example snippet that tags all AzureRM resources with the name of the resource group they are in.

Add-AzureRMResourceGroupNameTag.ps1

$tagKey = "resourcegroup"
$resources = Get-AzureRmResource

foreach ($r in $resources)
{
    $resourceGroupName = $r.ResourceGroupName
    $resourceTags = $r.tags
    if ($resourceTags) {
        if($resourceTags.Keys -contains $tagKey) {
            if($resourceTags[$tagKey] -ne $resourceGroupName) {
                $resourceTags.Remove($tagKey)
                $resourcetags.Add($tagKey, $resourceGroupName)
                Set-AzureRmResource -Tag $resourcetags -ResourceId $r.ResourceId -Force
            }
        }
        else {
            $resourcetags.Add($tagKey, $resourceGroupName)
            Set-AzureRmResource -Tag $resourcetags -ResourceId $r.ResourceId -Force
        }
    }
    else {
        Set-AzureRmResource -Tag @{$tagKey=$resourceGroupName} -ResourceId $r.ResourceId -Force
    }
}