Open tab in existing IE instance

You can use Start-Process to open the URL. If a browser window is already open, it will open as a tab.

Start-Process 'http://www.microsoft.com'

First you have to attach to the already running Internet Explorer instance:

$ie = (New-Object -ComObject "Shell.Application").Windows() |      Where-Object { $_.Name -eq "Windows Internet Explorer" }

Then you Navigate to the new URL. Where that URL is opened is controlled via the Flags parameter:

$ie.Navigate("http://www.google.com/", 2048)

Edit: In case 2 or more IE instances are running (additional tabs count as additional instances as well) the enumeration will return an array, so you have to select a particular instance from the array:

$ie[0].Navigate("http://www.google.com/", 2048)

You can use this if Internet Explorer is not your default browser:

Function Open-IETabs {    param (        [string[]]$Url    )    begin {        $Ie = New-Object -ComObject InternetExplorer.Application    }    process {        foreach ($Link in $Url) {            $Ie.Navigate2($Link, 0x1000)        }    }    end {        $Ie.Visible = $true    } }

I found this on PowerShell.com