PowerShell script to add user to SharePoint group

Add-PSSnapin Microsoft.SharePoint.PowerShell -EA 0

function Add-UsertoAllSiteCollections
{
    [CmdletBinding()]
    param
    (
    [Parameter(Mandatory = $true)]
    [String] $webApp,
    [Parameter(Mandatory = $true)]
    [String] $userName,
    [Parameter(Mandatory = $true)]
    [String] $userEmail,
     [Parameter(Mandatory = $true)]
    [String] $userlogin
    )
    $Webs = Get-SPSite -Limit ALL -WebApplication $webApp
    foreach($site in $Webs)
    {
      $Group = $site.RootWeb.AllProperties.('ADDCUSTOMLOGICTOGETGROUPDETAILS')
      $groupName = $Group + " " + "COMMONNAMEACROSSSITECOLLECTIONS"
      Write-Host "Site collection URL:" $site
      $groupName = $site.RootWeb.Groups | ? {$_.Name -eq $groupName}
      STSADM.EXE -o adduser -url $Site.url -username $userName -useremail $userEmail -userlogin $userlogin  -group $groupName.Name
    }
}
Add-UsertoAllSiteCollections -webApp "" -userName "" -userEmail "" -userlogin ""