LDAP query to test if user is member or group
DirectoryEntry rootEntry = new DirectoryEntry("LDAP://dc=yourcompany,dc=com");
DirectorySearcher srch = new DirectorySearcher(rootEntry);
srch.SearchScope = SearchScope.Subtree;
srch.Filter = "(&(objectClass=user)(sAMAccountName=yourusername)(memberOf=CN=yourgroup,OU=yourOU,DC=yourcompany,DC=com))";
SearchResultCollection res = srch.FindAll();
if(res == null || res.Count <= 0) {
Console.WriteLine("This user is *NOT* member of that group");
} else {
Console.WriteLine("This user is INDEED a member of that group");
}