Hello Lorenzo,
Huh... I suppose we don't have one. I can add this as a features request for future versions but until then... The only solution i can offer is not a very good one... BUT if you are in version 11x there is a call "GetAllMembersInOrganization" You could try to add your users to an organization? I have no idea if it would work... And like i said its probably not a good idea. But thats all I got :)