As we all know well, two months after the Enlightenment, on the full moon of the eighth lunar month, the Buddha preached his First Sermon at the Deer Park in Isipatana. The First Sermon is called the Dhammacakkappavattana-Sutta or the Setting in Motion of the Wheel of the Dhamma. On hearing this, Koṇḍañña, one of the five ascetics who had waited upon the Bodhisatta when he was practising self-mortification, gained the Eye of Truth (Dhamma-cakkhu), or the Wisdom Eye, as a first glimpse of Nibbāna. Koṇḍañña asked the Buddha for ordination and was admitted as a Bhikkhu, becoming the first member of the Sangha, or the Buddhist Order of monks. He is thus generally known as the Buddha's First Disciple. As until that time there had appeared in the world only the Buddha and the Dhamma, this event marks the completion of the Triple Gem of the Buddha, the Dhamma and the Sangha.
What should be noted here is the arising of the Sangha. Strictly speaking, it was the arising of the first member of the Sangha. That is, Koṇḍañña, who since then became known as Aññākoṇḍañña, was the first man to see the Truth after the Buddha and also the first to be admitted as a Bhikkhu.