I debated whether or not to share my one experience with a Meetup backpacking group (as it is a truly awful memory), but I will.
Early last summer, boyfriend and I were the only ones at a large lake (extending 3/4-1 mile from end to end), 11 miles from the trailhead, around 8k'. We'd already been there two nights and hadn't seen a soul since we'd left the car. The final 3-4 miles were still mostly under snow and lots of stretches were running streams. On a trial we'd traveled 5-6 times before, we had to check the map a number of times. I digress...
So, sometime mid-afternoon on our 3rd afternoon of having the lake entirely to ourselves and enjoying the peaceful scenery, three people show up. From a distance, they look pooped and drop their packs to sit and take a look around. We make ourselves visible and give a nod/wave. They come over and hiker #1 (who we soon discover is this Meetup's organizer) tells us they are waiting for the other 4 people in their party and then inquires about fishing and good camping spots. We tell him we haven't seen any fish (or people) and mention two different beautiful spots toward the other end of the lake - where we'd camped on all other previous visits. Meanwhile, #2 walks to the water's edge to take a look, and #3 walks by our tent and begins sussing out the area just beyond. As we keep chatting with #1, we can't help but notice #2 has joined #3, and the two of them are now kicking around and leveling the dirt about 50 feet beyond our tent.
At this part of the lake, you're actually sort of on a finger and the land ends about 100 yards beyond where our tent was pitched. This means, anyone camped beyond would have had to walk back past us for catholes, hiking, exploring, etc. Not only that, but there wasn't much (flora) between us and where it appeared #2 and #3 were thinking of setting up camp. We could see them clearly, as they stood around the exact spot where we had enjoyed the previous night's dinner and had spent a bunch of time lounging and reading earlier that very day. We called over... suggesting they check out other spots along the lake... They didn't even look up.
What came next was not pretty. In fact, it was entirely ugly. #2 and #3 retrieved their packs, #1 went over to where they'd been, and most of the rest of their group arrived. Eventually, as we stood dumbfounded... dismayed... while intermittently asking them to please not camp there (and saying some other much less pleasant things), they all made their way back over to the area beyond our tent. I pleaded, no, I BEGGED them to please, please, please, please camp elsewhere. I was practically in tears. I tried explaining we were there for solitude and to enjoy peace, beauty, privacy, each other, wilderness... to get away from "it all" and be alone. To this, the Meetup organizer curtly responded, "if you didn't want to be around people, then why'd you camp right next to a bunch of campsites?" Campsites!?!? What campsites!? We're in the w i l d e r n e s s... over eleven trail miles from the nearest car! This isn't a campground. Those aren't "campsites" - they are old fire-rings people didn't properly dismantle!
I couldn't believe it. We offered to help them move their things. We understood they were tired. We told them again about the other truly beautiful spots toward the other end of the lake. Our desperation was palpable. My boyfriend blew his lid (understatement). Basically, imagine the worst... now imagine something worse than that. No blows, but it was unreal. They were awful. We were awful. By this point, I had already dismantled our tent. We spent the next two nights at the other end of the lake. In the back of our minds, we worried they would come harass us or mess with our gear while we were out day-hiking. We also spent a lot of time trying to find some lightness in a dark experience that had left us with a bad feeling about humanity (ourselves included).
Two of the seven Meetup people were a couple who looked to be in their 50s. They were quiet the whole time and seemed uncomfortable throughout the entire ordeal. Even before they realized there was an "issue," they had hesitated to take their packs and walk by our tent. Eventually they did, but even then they sort of stood around as if they were contemplating not staying. My guess is that they shared our sense of wilderness "etiquette" and didn't want to camp there. Besides all the rest, at this point they understood how unhappy we were, and who wants to camp right by an angry man and a whiny woman begging you not to? It seemed to us this couple did not want to be a part of their Meetup any longer, but they were stuck... not realizing until then that their organizer was a jacka** or the temperament of others in the group.
So, be forewarned. You never know who's going to be in your Meetup group, so you might want to have an escape plan in place. And I would advise against carpooling with/to join a bunch of strangers for a multi-night backpacking trip; you could end up between a real rock and a hard place.