How to get all email IDs from an outlook group ID in ASP.Net

"outlook group ID" = distribution list?

You can query Active Directory, searching for "CN=DL_name"

SearchResult r = SearchObj.FindOne(); 
foreach (string user in r.GetDirectoryEntry().Properties["member"]) 
Response.Write(user);

In client tools you can also use Microsoft Outlook Object Library component.

http://msdn.microsoft.com/en-us/library/bb645998.aspx