From Secrets of SharePoint -  Tip of the Day

When creating Web site using PowerShell, you may encounter situations where you’re missing the site’s default groups. A simple solution to this is using the CreateDefaultAssociatedGroups method. The example below demonstrates how you can use the method:

PS > $spWeb = Get-SPWeb http://SP01.powershell.nu

PS > $spWeb.CreateDefaultAssociatedGroups(“powershell\spAdmin”,”",”")

PS > $spWeb.Update()

PS > $spWeb.Dispose()