If you are even considering this, do it under the optimum circumstances. The what-if article describes it nicely. In most practical experiments its just not feasible at all.
However, there is the situation of the new moon, and the availability of beam expanders. Combining the two, and using a billion people at the same time, 1 watt green dpss lasers of good quality, and 10:1 beam expanders, it would be possible to create a visible greenish spot on the new moon.
If the moon is even slightly lit by the sun this would probably no longer be visible. We are up against a light source that puts out a whopping 4E+26 watts, a billion billion billion laser pointers. It IS a lot further away and radiates in all directions to we would not even have to get close to that order of magnitude. Putting a visible laser spot on the full moon is, however, still something far beyond what humanity could achieve, even if we dedicated all power generation capacity on earth to attempt it.
So, is that the end of it all?
Not really. There are some retroreflectors left on the moon by the apollo missions, which are still used to do lunar rangefinding experiments today. It would be an enourmous achievement to somehow produce a visible reflection from a retroreflector, even when looking at it using a telescope that would be the size of a bus. Achieving that is a very ambitious goal, but more realistic then 'lets have a billion people point a laser pointer at the moon and see'.
The lunar rangefinding experiments use low divergence beams to operate, and the return signal is basically a countable number of photons per second - not enough to see even with a telescope.