The word Sangha refers to the Ariya Sangha as well, not just the Bhikkhu Sangha. It is possible for a lay person to be part of the Ariya Sangha see http://en.wikipedia.org/wiki/Sangha I'm not convinced that the Bhikkhu Sangha is the "only authoritative Buddhist teaching organization", it may have been 2500 years ago but Buddhism has adapted and evolved since then.