Ok, so I chose Atom (had to be a GitHub product, right? ):
The most obvious evidence is from the Pulse overview for the past week, 13 pull requests closed, 45 issues closed. Decent numbers.
The second piece of evidence is from the Contributors view, but not the contributors themselves, but rather the commit group which shows the consistency of commits over an almost seven year period.
The first of these is likely to be more useful in student work than the second on a small scale, but for me the mist useful part of using git(GitHub) for student group work is to show that they are working consistently thought out the project lifetime and not just cramming in the work at the last minute.