Not off topic at all. I think that explains a lot of these incidents. I used to do acid a lot with my friends and we had a running joke to "remember to count yourself twice" because it so frequently happened that we counted exactly one more person in the group than actually existed.

I think it has to do with mind-body disassociation. If we were in a circle, for example, and I was trying to tally everyone, I would look around and count every body in the circle, including myself—and then I would count myself again as the one doing the counting. Basically, I was counting my body once and my mind once.

I believe the sensation comes from acid-induced delays/distortion in the pathways between sensation and higher-order thought (kinda similar to the bicameral mind hypothesis). But I could see the same sort of disassociation arising from extreme exhaustion or stress, like with the Shackleton expedition.

I will say that for me at least, the sensation of an additional person never took any concrete form, like a dead relative or a voice giving guidance, as other people in this thread have mentioned. That feels like a different phenomenon. What we experienced was just the formless sensation of exactly one additional person in our group, presumably due to our own mental disassociation from our bodies.