How do I set a proper search filter for an AD Directory search?

DirectoryEntry root = new DirectoryEntry("GC://dc=abc,dc=com");
DirectorySearcher searcher = new DirectorySearcher(root);
searcher.ReferralChasing = ReferralChasingOption.All;
searcher.Filter = string.Format("(&(objectClass=user)(memberof={0}))", group);
var results = searcher.FindAll();
              

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