Indeed, as others in this thread have noted, you sure can just do that.
Granted, as they've also (mostly) emphasized, it's a pretty horrific process getting--and keeping--the bitter, slimy, vomit flavored blender-full of thick green cactus snot in your belly. Having done just that, lemme just tell you that it's not recommended. And that's coming from someone who has imbibed some truly foul tasting substances without much of an issue. Trichocereus smoothie? Maybe not so much. As in, never again.
The experiment in question took place years ago. And, basically, we were impatient and wanted to get the San Pedro show on the road, so to speak. The cactus, by itself, was too thick to get it completely blended. It just turned into a slimy, chunky, part blended ball of cactus goo at a certain point. We proceed to add just enough liquid (a little bit of water and fruit juice) to get it to blend up, which proved to substantially increase the already disturbingly large volume required for each dose. Also, it made every single drop of the additive taste JUST as bad as the original ingredient. It lent it's inimitable flavor/texture to each and every added milliliter of the "liquid" end result. (I still shudder thinking about it.... Seriously.)
Anyway, to make a long (and excruciatingly nauseating) story a wee bit shorter, five of us managed to choke down between a full half (two people) to an entire blender carafe-full (three of our number--yours truly included. Each. And it definitely did work, I'll freely admit.
But, as I lay on the couch all night clutching my stomach whilst plunging through an abyss of glorious mescaline visuals, I decided that having stabbing pains, cramps, and nausea--literally all night--probably wasn't something I ever want to have to repeat in order to have that level of experience. Nonetheless, I was still able to appreciate how lovely the trip turned out, despite my extreme physical distress. That in itself probably tells you something about how lovely the cactus can be.
All in all, it's a wonderful, benevolent plant ally. Truly good "sun medicine" (even though I like to take it at night).
My suggestion is just to use WAY more plant material than you think you'll need for each person, to start off with because it often takes A LOT. Also, do yourself a favor and plan ahead. Prepare it for consumption by reducing the mass that you have to ingest, because it's pretty challenging to consume in almost all of its forms.
However you do it, when you do it-- if you do it--be safe, have fun, and most off all enjoy it!