You can use the LookupAccountSid
function to do that:
BYTE bBuffer[128];
DWORD dwSize = sizeof(bBuffer);
if (CreateWellKnownSid(WinBuiltinAdministratorsSid, NULL, (PSID)bBuffer, &dwSize))
{
wchar_t wchName[128], wchDomain[128];
DWORD cchName = _countof(wchName), cchDomain = _countof(wchDomain);
SID_NAME_USE use;
if (LookupAccountSid(NULL, (PSID)bBuffer, wchName, &cchName, wchDomain, &cchDomain, &use))
{
// wchDomain will now contain something like BUILTIN
// wchName will now contain something like Administrators
}
}