<p>Currently we have a DLL that checks whether a username/password is a valid Windows user using the Windows API LogonUser method. We need to enhance it so it checks whether the user belongs to a specified group as well. Is there a Windows method that does that?</p>

<p>Given a Windows username and password, find out whether the user belongs to a specified group.</p>

The closest single API is
CheckTokenMembership
, which just leaves the problem of obtaining the user's SID and handle for the group.
If you can use ATL check out the methods on the
CAccessToken
class ... and even if you can't use it check out the implementation in atlsecurity.h for examples.