public extern static int NetUserGetGroups

        Public Shared Function NetUserGetGroups(

        public extern static int NetUserGetGroups

            ErrorCode = NetUserGetGroups(ServerName,Username,Flags,out bufPtr,1024,out EntriesRead, out TotalEntries);

The prefmaxlen parameter is DWORD, changed it to UInt32. This was already done for the netusergetgroups http://www.pinvoke.net/default.aspx/netapi32.netusergetgroups. Otherwise MAX_PREFERRED_LENGTH could not be passed which is defined as (DWORD)-1.