Sorry for the long post.... TLDR... Most people are awesome.... but VET VET VET the people you invite...
I learned to vet the fuck out of strangers before inviting them the hard way on my 2018 Grand trip...
We actually airlifted a crazy dude out that I had warning bells and red flags waving from the moment I met him. I even asked the PH she really wanted this dude on the trip before we left. He had his stuff strewn all over the airBNB and was smoking weed in a non-smoking house while everyone else was out working on getting ready for the trip. He was already arguing and being a dick with the TL (different then the PH) on rig day. Someone brought a boat for him since he was a poor dirtbag day trip guide and just couldn't stand anyone telling him how to rig the boat and was gonna make sure they knew it.
He was always throwing empty beer cans on the floor of the raft, wouldn't put the oars in the water and was always a mile behind the rest of the group. We had rough time on a long day made worse by all the camps near Nankoweap being taken plus most of the ones downstream and then he missed the eddy at camp once we actually found one. The kitchen was on his boat and he refused to help haul his boat up to camp (easy pull up a beach around a few rocks) and got into a yelling match with the guy who's boat he was carrying and then proceeded to talk shit about him all night and was talking about how he was gonna kick his ass. Other dude found out about it the next morning and kicked his ass off the boat and let someone else row it and made him ride with the PH who invited him.
Further down...near Blacktail... after dealing with his lazy ass, someone on his meal team went down to have a "come to Jesus" talk with him to get him off his ass...and dude flipped out and started attacking the guy, who was twice his size, and then had a full on temper tantrum mental break where he was throwing his shit in the river and began threatening to kick everyones ass, hurt himself, and all manner of other BS before stalkin off downstream and away from camp (across from Blacktail...basically cliffed out and nowhere really to go). At this point, basically everyone on the trip was uncomfortable with him being on the trip so we called dispatch with a Sat Phone and they agreed that it was probably just gonna keep escalating and agreed it was time for evac. It was too late for them to come safely that day but they said they would fly down and figure it out at 8am the next day. We didn't really see him that night, but he kinda came in and got coffee the next morning and asked what was going on but didn't know that his fate had already been decided till he heard the helicopter....at which time he proceeded to sling his coffee mug at the nearby cliff and have another bout of angry ranting.
A couple LEO rangers stepped out of the little Bumble Bee Helicopter the NPS uses in the Grand and proceeded to split up and one talked to the TL and PH and the other the Crazy dude. Needless to say, after hearing what all went down, they agreed that it was time for Crazy dude to no longer be in the Grand and that they would take him out of there. They weren't allowed to cuff him and said he would have to come willingly but that it would be best for us to move on downstream so he wouldn't have any other options. We asked what happened if he wasn't willing...and their response was "we love riding in the helicopter...we'll just leave him here for a night and come back in the morning and see if he's hungry". Needless to say...the rest of the trip, even though we had the usual drama of boat flips and other issues you see in the canyon, was much more pleasant from then on.
There was a thread a while back about a trip from hell where one of the trip members turned out to be kinda sociapathic and decided to "have fun" by deliberately going out of his way to manipulate people on the trip to get into arguments with each other. LIke... the dude just wanted to see the trip fall apart and watch the world burn around him kinda stuff. Talking shit about one person with another, then doing the same the other way and just plotting everyone against everyone else. They eventually figured out what he was doing and basically ousted him from the trip but not before it basically ruining the trip for everyone. I've heard of multiple cases of trips not getting along and one half of the trip not talking to the other half...to the degree of one half being on the upper beach the other half on the lower and the kitchen in the middle and taking turns cooking and eating. I forgot who it was... but there was one trip that was so bad that the Rangers gave them permission to split the trip up and camp in different camps. They had enough of everything for both halves to be able to get down the river and make food.
I do think these are exceptions and not the rule though. Most trips are amazing and everyone, for the most part, gets along swimmingly. That has been my experience on the 7 Grand trips I've been down so far. I've been on many many other multi-day trips and have only met a handful of people have made the "do not go on a trip with" list.