Finding out if user is a member of Active Directory Group can be done using following snippet of code
Method 1: using PrincipalContext
public static bool IsGroupMember(string domain, string group, string login)
{
bool result = false;
PrincipalContext context = new PrincipalContext(ContextType.Domain, domain);
UserPrincipal userPrincipal = UserPrincipal.FindByIdentity(context, login);
...
