How did early Hindus believe one could achieve union with ultimate reality?

It was the duty of the individual self (Atman) to seek to know this ultimately reality. Hindus developed the practice of yoga, which was a training designed to lead such union.

Hindus practice a lot of yoga back in the days. They did a lot of meditation. Some even say that some sages did so much meditation that the god him self appeared in front of him and gave him booms. With this yoga came abilities to the sages. They achieved the ability of 6th sense.