I've done PBP and LEL once and never entered them with an intention of riding with anyone in particular.
On LEL I ended up riding with a YACFer within 20km of the start and we rode the entire rest of the thing together. There were a few times when I waiting for him whilst he slept and I could of continued and there were (certainly more) times when he had to deal with me messing up my sleep and having to wait for me whilst he could have continued. If at any time I'd thought I was really holding him back (especially if it could have jeopardised his ride) I would have told him to go on, I probably did make that offer several times and he stuck with me and helped pull me through my bad patches.
On PBP I rode most of the first 24h on my own (although a few hours with one YACFer), then hooked up with one fellow Brit for 36h or so, then another YACFer for 24h or so, plus a couple of other YACFers between two controls later on, but did a good day and a bit completely on my own. Most of the arrangements fell apart at a control where we'd have different expectations and plans ("I'm going to bounce it", "I've got to sleep", "OK, thanks and good luck, enjoy the rest of the ride", etc). There's so many people around on PBP that you can find a group (and some words in a common language) to see you through to the next control or beyond.
Planning on riding in a specific group from the start could easily go wrong for so many reasons, but it depends on the people, I've known groups who've done it successfully with no problems. An understanding of what happens if people do start to really lag behind and jeopardise the rides of the others needs to be discussed in advance.
I know I've "accidentally" lost someone I was supposed to be riding with in order to save my own ride.