Example to add an user into the SharePoint Group using STSADM Command

Logon to SharePoint Server which is hosting the SharePoint Site.

Go to Command Prompt as Admin

Now run the below command and provide details as mentioned below. SiteCollection Admin UserName should also be like Domain\UserName

stsadm -o adduser -url "SharePoint Site URL" -userlogin "domain\username" -useremail "username@domain.com" -group "group name" -username [SiteCollection Admin UserName]