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I am not sure I am understanding this question properly, and don't know if your meaning 'group' as in mediums collectively and in general, or if you mean when a 'group' all see the same thing at once. I also don't know what datura is, but surely you don't think all mediums are taking drugs (coz they certainly are not) so we'll go for some other explanations/clarifications.
Clairvoyance happens both objectively and subjectively.
Subjective, which is the most frequent way, is basically seeing an image in your mind the way you would if you deliberately tried to imagine/picture something, tho it is usually a lot more vivid than it would be if you tried to picture something deliberately. Sometimes it is like glimpsing snap shots, sometimes it is like viewing mini film clips. If tuned in together, it is possible for more than one medium to see an image.
Objective is also done with the mind, however the image appears to be projected in to your external field of vision so appears in front of you. Barring a small proportion of incidents where I have seen spirits surrounded in blazing light, they generally just look like exactly like people do, ... solid, full color, animated. To me they don't slowly manifest before my eyes, they are just there in a split second. More than one medium can see an objective image at once too.
Manifestations as seen in physical phenomena, do not require clairvoyant vision and can be seen by everyone present medium or not, ... and none of these types of incidents involve anyone taking any drugs or other substances. Of course I can't vouch for every person in every incident, so of course some individuals may have done that, but it is not a standard practice, you don't rock up to circle and pass the drug plate around (lol), and when others who have not, or more likely - everyone who has not, can see the same thing anyway, it is not as tho it can be classified as a drug fuelled mass and shared vision.
Hope that answers your question