You can Add user on SharePoint’s Group using Powershell, to do that you just need to know two things:

• User domain name -> domain\User
• Group’s name – you can check this on the following post – http://wp.me/p4Hadm-45

Then you can run the following powershell script:

$site=get-spsite http://fcsharepoint

$web=$site.RootWeb

$web.SiteGroups |select name

$user=$web.EnsureUser("LOCALDOMAIN\FCShareUser")

$group=$web.SiteGroups["FCSharepoint Members"]

$group.AddUser($user)