The difference is just that. One is a hallucination and one is a real spiritual experience. What you are questioning is our ability to distinguish between the two. This is where reason comes in. The quick and simple test is to evaluate the fruits of the experience. Did the experience glorify God? Did it bring about some good that is easily understood as being compatible with God’s will? Then it very well could have been a real experience.
Yes, they all see the same things with their minds. Is that really such a bizarre concept? If a single person’s brain can receive and process spiritual “information” (for lack of a better word) which produces the effect of “seeing” a spirit, then why couldn’t multiple people receive and process this same information?
There was a study years ago where a doctor used electromagnets to stimulate people’s brains so that they would experience various sensations that felt like spiritual phenomena to the subject. The goal of the study was to show that spiritual experiences are generated by the brain and completely explainable by psychology. The problem with the conclusion is that the study only showed that our brains can be manipulated by external sources in ways that alter our perceptions, making us see, hear, and feel things that aren’t “there” to the usual five senses. So one could come up with the exact opposite conclusion of what the doctor concluded, in that it showed that our brains are capable of non-physical perception. So if spirits exist, then this study showed a possible method of how we could perceive them.
Back to your question of whether a group of people could all “see” the same spirit, just consider the fact that a spirit is not physical, so not bound to space. In other words, it’s not a body standing in front of the group of people for them to “look at”, but rather an intelligence that could “communicate” with all of them, seemingly at once. So sure, they could all “see” (ie, receive the same “transmission” of information from) the same spirit.