How to get logged in user's Group Name(s) by coding?

Hi, I am using  Windows Authentication in my ASP.Net Website Project.
I need to get logged in user's Group names, Can someone tell me how can I get logged in user's Group Name(s)?
I have to check if user is member of a specific role then only I have to allow him to log in to the site. Now I know that I can use User.IsInRole method but I donot know the exact Domain Name to be passed in this method. Because my company has a global network all over the world, Anyone in our Network can log in to this application if he is assigned to a specific Group/Role. 
Lets say, If user logs in from Usa, the role can be "CompanyUS\SpeficRoleName", if person logs in from UK, the role can be "CompanyUK\SpecificRoleName" etc. So first of all I want to get all the roles or groups assigned to the logged in user. How can I get this list?
Here is my config file.
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