The thing is that members of a directory service group are added only if they have logged in to Tridion at least once. If you want to browse the directory service group users you need to get the path of the active directory group (subtree, leaf) by looking at:

 GroupData group12 = (GroupData)_client.Read(subGroupId, new ReadOptions());

group12

The thing is that members of a directory service group are added only if they have logged in to Tridion at least once. If you want to browse the directory service group users you need to get the path of the active directory group (subtree, leaf) by looking at:

 GroupData group12 = (GroupData)_client.Read(subGroupId, new ReadOptions());
 
 var searchDn =((DirectoryGroupMappingData) group12.ClaimMappings.First()).SearchDN;
 var directoryServiceName = ((DirectoryGroupMappingData) group12.ClaimMappings.First()).DirectoryServiceName;
 var result = _client.GetListDirectoryServiceGroupMembers(directoryServiceName, searchDn);

I can't test it at the moment, but it should give enough directions.

Keep in mind that code assumes that there's just one ClaimMapping in the group and that it's of type DirectoryGroupMappingData, which is not always true