Waking transition
As dreamers become more and more attuned to Oneira, they eventually slip into it entirely. For many, this occurs while sleeping, as their dreams entangle with the dreamworld. Some reach the point of transition while awake on Earth. The disappearance of such individuals is typically more dramatic than that of a sleeper. A waking candidate who is flying a plane might leave it to crash mysteriously into the sea without a pilot.
Sleeping transition
Moving to the dreamworld while sleeping is probably less of a shock. The sleeper wakes to discover the world is different, and may even forget enough of the ways of Earth to accept her new locale. As though waking from a dream, and shedding the fog of dream, she might come to disbelieve some of her past. It was all a dream. This is real.
For others, though, waking from a dream to find themselves still within a dream might become a sort of endless waking nightmare. They keep trying to escape the dream, and can only awaken again still trapped by it. Such unfortunates may go mad, and this can be dangerous to those around them when their dreams reflect that madness.
Displacement
The world is not decreasing in mass. (Or maybe it is, if that is a theme of your game.) This makes no sense if dreamers are occasionally disappearing to another dimensional space. Perhaps the fall of stardust, literally with meteoric showers, offsets their loss. Perhaps some like mass of matter from the dreamspace transfers to Earth to offset their displacement.