Are there other people, according to Buddhism? When I was on a Self Inquiry retreat last year, I was explaining why I wanted to help other people. One of the two teachers looked at me and said, "There are no other people." That sort-of made sense to me. The next day I had a realization which made it make sense completely.

But the other day (is there another day?) I saw it in a different way: that it simply does not matter whether there are other people or not, because I can never have real insight in to them, so I am completely alone in my path. My image of two people is like spheres touching at one hypothetical point (Indra's Net). I cannot see much through a point-sized window on to (hypothetical) someone-else's world. This is like Vivekacudemani #515:  


  I am Actionless.
  I am Changeless.
  I am Partless.
  I am Without Purpose.
  I am Eternal.
  I do not require any prop.
  I am One, without a Second.


If I am One, can I know anyone else? What does Buddhism say on this point?
The practice of a bodhisatvva
  
  The entire practice of a bodhisattva is divided into six perfections, which are generosity, discipline, patience, effort, meditative concentration, and wisdom.
  In order to fulfill the hopes of others, it is very important to engage in the practice of generosity, which in turn should be strengthened by a strict observance of ethical discipline, that is, of non-violence.
  The practice of discipline must itself be complemented by that of patience, because you will need to have forebearance when facing the harm inflicted on you by others.
  In order to succeed in these practices, your endeavour must be strong and stable.
  And without meditative concentration that effort will not be so effective.
  Finally if you do not have the wisdom that realizes the nature of all phenomena, you will not be capable of guiding others on the path that leads to enlightenment.
  
  The sweet taste of bodhicitta
  
  If there is one practice that is sufficient to bring about buddhahood, it is the practice of great compassion.
  Chandragomin, a sixth-century Indian poet, said that it is stupid to expect to change the taste of a very bitter fruit by simply adding one or two drops of sugar to it.
  In the same way, we cannot expect the fragrance of our minds, which are so contaminated by the bitter taste of delusion, to change instantaneously into the sweet taste of boddhicitta or compassion on the basis of just one or two sessions of meditation.
  Sustained and continuous effort is extremely important.
  
  As the Buddha himself said, on th strength of their wisdom bodhisattvas abandon all delusion, but through the vigour of applying compassionate methods, they never abandon beings.


The Dalai Lama's Little Book of Inner Peace -- The Essential Life and Teachings



There was also a video in which the Dalai Lama said,


  I never consider myself as something special. If I consider myself as something different from you, like I am Buddhist, or even more, I am His Holiness the Dalai Lama, or even if I consider I am Nobel Laureate, then actually you create yourself as a prisoner. I forget these things, I simply consider I am one of the seven billion human beings. We are mentally, emotionally, intellectually, we are the same.


He goes on to say that people get fulfillment of purpose, and "always feel happy", from helping others, etc.

It's not that other people "don't exist": it's that there are not "other" people, not different; i.e. "we are the same".