Add-PSSnapin "Microsoft.Sharepoint.PowerShell"
$web = Get-SPWeb "https://Marketing"
$userObject = $web.EnsureUser("ABC\a.b")
$group = $web.SiteGroups["Site Members"]
$group.AddUser($userObject)
By Indra
SharePoint Architect
Add-PSSnapin "Microsoft.Sharepoint.PowerShell"
$web = Get-SPWeb "https://Marketing"
$userObject = $web.EnsureUser("ABC\a.b")
$group = $web.SiteGroups["Site Members"]
$group.AddUser($userObject)
SharePoint Architect