GetUserName doesnot give full username

I am using windows 7 and pb 11.2. When I am using

When I am using GetUserNameA( ref string userID, ref ulong len ) library "ADVAPI32.DLL", it doesnot give full userid. for example my userid is maingroup\1024 but getusername only shows 1024 , ignoring the mailgroup. Please can you advise me Regards Pol