Can anyone recommend let's play channels or streamers wherein a group of multiple people play single player games together?

They play single and multiplayer games together, but the Runawayguys. They mostly do local multiplayer games, but recently they've done some specifically single player games like the Metroid prime remaster.
- Sounds good! Thank you. I'll give them a try

I like RKG! They play Dark Souls and those types of games- one of them plays, the other two chat about the lore of the game or whatever. Right now they’re playing Elden Ring!

How big of a group are you looking for?
mikeburnfire is pretty fun.
Most of the videos consists of two friends, Mike and Zack, playing modded Bethesda games. Their main gimmick is that they are playing games like Fallout NV in "multiplayer" but they also play other stuff (Mass Effect, Inscription, Stray to name a few).
Lot's of military stories, some great gun rants from Zack and a general feeling of just two friends having adventures together.
Over six years of content and still going strong.
- Any size group is good! Thanks for the suggestion - sounds good. I'll give them a try :)
  
  I also completely forgot to mention another duo - Chip & Ironicus.
  This one is a little different as they record their commentary over premade footage. They can also be a bit more... low energy (?) compared to many bigger channels but if there's one thing they (or more specifically Chip) really shine in, is the huge amount of effort put into each and every playthrough.
  Pretty much every episode is released in two versions, cut and uncut commentary, to let you choose whether you want the option to focus during cutscenes etc. They are also filled with cool trivia, secrets and just good skill level in general.
  They have a pretty legendary MGSV LP where Chip prepared 3 seperate playthroughs, each with a different approach (stealth, combat and goofy).
  Besides that, they also have stream recordings with live commentary, a podcast and regular charity streams.
  Chip worked at recently closed Volition so he could definitely use more views.

If you like Game Grumps, maybe check out Oney Plays.
- Will do! Thank you for the suggestion :) I hadn't heard of them before.

How about co-op multiplayer? I've seen a few of those for Baldur's Gate 3. Like Neil Newbon or Jennifer English. Neil plays with his friend Tom (whose surname I have forgotten) and Jennifer with her girlfriend Aliona Baranova. Except for Tom all of them acted in the game.
- I do like let's plays like that too sometimes - I'll give those channels a try. Thanks! Though I may have to wait til I finish with BG3 myself, which could be a while :p
  I mentioned single player games specifically partly because I personally tend to like those games best, and I like to watch let's plays after playing the game through myself first, then seeing how different people interact with the game differently. I love watching people discover a game I enjoyed (which for me means mostly single player titles) in kind of the same way I might enjoy showing the game to a friend.
  And anecdotally, I tend to feel like groups playing a single player game together tend to talk more about the game in a deep-read kind of way, or to talk about their lives, whereas groups playing multiplayer games seem more likely to talk about whatever is currently happening in the game in that instant, or it becomes mostly them joking and trolling each other. This is just my personal experience though, so it could be a function of the particular let's players and streamers I'm familiar with. I'm sure there are exceptions to this.
  
  I like watching Let's Plays for the same reason as you, and while it isn't a group channel I heartily recommend Welonz. She is very thorough and approaches every game with great respect, usually giving a summary of her overall thoughts at the end.
  I found her because of her VTM: Bloodlines LP, but her Witcher 3 and Cyberpunk ones are also fantastic.

Loading Ready Run streams a lot of stuff. And you might like the dessert bus for hope fundraiser happening in November.
- Specifically I would say the following streams fit the bill:
  Let's Nope (horror)
  Let's Watch (Steam garbage)
  Rhythm Cafe (rhythm games)
  Talking Simulator (hard to describe)

Does girlfriendreviews count?

PlayStation Access have some really nice let's plays. The Skyrim Christmas special is amazing!

If you like old adventure games, ps_garak on twitch often streams with his friends

Check out "Continue?" on youtube they just find an old or obscure game and play it and rate it continue or not.

Not quite the same thing, but one thing I have seen is players that stream slower-paced games chatting with remote viewers.
On Cataclysm: Dark Days Ahead, Vormithrax, is well-known for this, and watching his videos has often been recommended on Reddit as a way to learn the (quite complicated) game, as he tends to walk people through what he's thinking about while playing.
Obviously, that doesn't work with every game genre; they have to be able to field suggestions and questions from viewers while concurrently playing. But for turn-based games, I think that it can work well.

If you take a little time to get into them, the early "Let's Game it out" videos are hilarious. e.g. https://www.youtube.com/watch?v=1RPEtoJ9ZA8

Well, not dedicated to that, but mr samuel streamer (mostly crusader kings and rimworld) has an alt channel where he shoots the shit with his fiance, and sometimes they play games together. They're playing vampire: the masquerade: bloodlines rn.

Worst Premade Ever (WPE)
They are British, one of their editors is from New Zealand. They threatened to replace their editors' livers with live badgers if they unionized. 10/10 would watch the surgery.

You could watch best friends play. They dont make new content but if you havent seen any of it there is plenty to watch. Sad they are gone though

If you're into pro wrestling, there's the Newlegacyinc boys, they do playthroughs and streams of wrestling (and wrestling adjacent) games. I'm not even really into wrestling and I still find them very entertaining, their streams of MDickie wrestling games are top tier.

Jelloapocalypse plays games with his friends. He also has highlights up on his main channel if you'd prefer something more edited.

If you don't mind a group of friends playing multiplayer, there is Stumpt. Depending on the game they're playing, you either get some commentary about the game or in the case of videos like their Game of Life 2 videos, they kinda just talk about whatever because they've played it so much.

Check out neebs gaming on YouTube. Excellent and very funny. I also highly recommend their subnautica series, not to give too much away but it follows a different script than the story in the game. It is amazing, and the quality of their videos is top tier.