The actual value of the assignment group can be achieved by referring assignment_group.parent.parent.Not sure if I can use Glide Records but I am using Visual Studio 2010 to connect to web service and I am not able to add the required class for GlideRecord.
Is this a question about reference fields in ServiceNow, or about using Visual Studio with ServiceNow?
Assignment group isn't a derived field, it's a reference field. Are you asking how to get a derived field off of the sys_user_group table, by dot-walking through the assignment_group field? There isn't any trick to it. What kind of script are you trying to do it with? If it's a client-side script, you might be running into a problem because you can't dot-walk on the client side; you'll need to make a GlideAjax (or, if performance is not a concern, GlideRecord) query to the server.
Please be more clear with your questions in the future. We cannot help if we can't decipher what you're asking.