A runner (OK, it was my son, who is a junior on the team I coach - since I'm the frosh-soph coach, I don't directly coach him any more) asked me the other day, "Was I running with the right guys yesterday in the speed workout?" This was a more complex question than it seems.
My answer had a few different parts. First of all, as I noted in my post about running repeats, you want to run all of your repeats at about the same time. But not slowly at the same time: you should be pretty darn exhausted at the end of a workout of repeats. So the first part of my answer was a question: Did you run evenly, and were you exhausted at the end?
Next, I asked another question: Which guys on the team do you want to (or think you should) be beating in races? This is tricky because there may be a guy you see who you think you should be able to hang with, but he may actually be faster than you can run right now. But using your teammates to gauge how hard you should be working can be a good way to improve.
What you don't want to do is pick some guy that you want to run with, get on his shoulder, and run the first one or two repeats way too fast. That would put you in a situation where you don't run evenly. But you might try hanging with him for part of the repeat. For example, let's say you pick out "Jim" as the guy you want to run with. When you start, get on his shoulder for the first 100m or so. Is he going too fast? Let him go. When you get to, say, halfway through the repeat, take note of how far ahead of you he is. For the remainder of the repeat, maintain that distance. When you finish, assess how you feel, then make adjustments on the next one.
The bottom line is that chasing down runners who are faster than you (in speed workouts - don't do this on easy days!) will probably result in improvements in your own running. Like everything, though, you have to experiment a little to make it work.