Expand domain group

Hi,

Is there any API able to 'expand' a domain group?. I.e.:

1. I've created a Security Group (say, 'GRP1'), added some users into it;
2. Do UserGroup.asmx->GetUserCollectionFromSite call, the GRP1 is present in the returned result;
3. Try to do UserGroup.asmx->GetUserCollectionFromGroup("GRP1"). Result: no such group found (expected, because GRP1 is not a SharePoint group).

I need to somehow retrieve a list of members of GRP1, but do not see any public interface to carry this out. SharePoint Online PowerShell cmdlets are able doing this, but they are calling some 'provisioningapi' that seems not to be documented.

Any suggestions?