This same multitude of beings, being born again and again, dissolves at the coming of night, oh Pārtha!, involuntarily, and arises at the coming of day.

Bhūta-grāmaḥ — multitude/congregation of beings. Sa eva ayam — this same (cyclical repetition). Bhūtvā bhūtvā — having been born repeatedly — pralīyate (dissolves).

Avaśaḥ — without own will, involuntarily. The jīva do not control the cosmic cycle. They are carried along by prakṛti and its time. Only mokṣa breaks this cycle.