Check a collection of SIDs for membership in a given group

1. Use LookupAccountSid to get the account for the SID
2. Use NetUserGetGroups to get all groups this account is a member of.
   EDIT Note from eryksun: Check target group's SID first. If it's SidTypeGroup, call NetUserGetGroups. If it's SidTypeAlias, call NetUserGetLocalGroups (AKA aliases, such as "BUILTIN\Administrators")
3. Use LookupAccountName to get the SIDs of the group.
4. Check if account SID (step 1) is member of the SID collection (step 3). Use EqualSid function.

You find appropriate old code one the old win32.mvps.org pages. Sadly the page is gone. But Wayback machine helps. Here a link to the old fksec samples

You find an easy to use sid class there. It also provides sid::MemberOf and sid::Members functions. The code isn't my style but it works.