Use Concurrent Collections

/** thread-safe */
class Crew {

  private final List<String> members = Collections.synchronizedList(new ArrayList<>());

  void add(String name) {
    synchronized (this.members) {
      if (!members.contains(name)) {
        this.members.add(name);
      }
    }
  }

  int size() {
    return this.members.size();
  }
}

using atomic methods with a thread-safe data structure may still lead to wrong results
synchronized sometimes still necessary for typical operations on a list (check and modify)

/** thread-safe */
class Crew {

  private final CopyOnWriteArrayList<String> members = new CopyOnWriteArrayList<>();

  void add(String name) {
    this.members.addIfAbsent(name);
  }

  int size() {
    return this.members.size();
  }
}

make that atomic which should be atomic